I wonder why the UK used so much low pressure sodium lighting compared to the rest of the world? I wonder why the rest of the world was more reluctant to adopt low pressure sodium lighting and why the UK was more willing to adopt low pressure sodium lighting?

It is "british invention", so for Brits it would always be better than anything invented elsewhere.

The rest of the world just asses all aspects:
Expensive, monochromatic so no color rendering at all, need ballasts that are very inefficient (until the electronics alowed to build at least the dedicated ignitors), very large for the light output so impossible to utilize more than fraction of the light so fixtures with other modern technologies were not that far behind and today are even ahead in the race for the lowest energy consumption.
While the only advantage isthe high brutto efficacy of the lamp, when no efficient beam control would be used anyway.
And the "no mercury" green hype (ignoring there are other toxic materials used in the lamp anyway)

What's toxic in a SOX lamp..? Sodium possibly, but unless you go around eating it I don't see a problem.

Have you ever driven under LPS lighting..? Far better than anything else I've ever seen on the road. Colour rendering is not important; any of that you need is provided by your headlamps. Size of the lamp - yes they're large but as a consequence they don't appear as glare bombs the way smaller lamps do.

Back in mid 70 to 1980s  US had way more sox lighting back then due to  1970s energy crisis,  lot  of the post office,  some cities,  and bussiness ran sox lighting in those days.   Problem was lamps were expensive to replace so lot  of them got replace  in the 1990s for cheaper HPS.  Then there places that kept them like observertory,  I know a public storage unit had 35 watt sox wall pack, until they could not get the lamps any more. 

Another reason it seems it lot easier to get older stock of sox lamp in the US than u find them in the UK and when u facture in postage to ship from the UK.

 I have found Norelco, Verd A ray, philips, GEC super sox, GE and osram, GEC did sold 10 and 18 watt Nightwatch, Sox 55er( beta 2) Thorn alpha 4,  philip gold finger, AEL sox lantern, and palmar/spaulding sox lantern,  and Hubbell wall packs in the US.  Even Norerelco made a sox yard blaster with double refractor glued together. 

So Dont under under esimate what US did have and then have
Very low number existing in the end.  @ WorldwideHidlamps

My evidence for low pressure sodium lighting being so popular is that there were many fixture designs for low pressure sodium lamps for the UK market and there were several types of low pressure sodium lamps for the UK market aside from SOX. Those lamps were SLI/H, SO/H, SOI/H. In the US, we had fewer lamp designs for low pressure sodium lamps. They were the NA type lamps that had been used for street lighting and the imported SOX lamps that came from the UK.

Back in mid 70 to 1980s  US had way more sox lighting back then due to  1970s energy crisis,  lot  of the post office,  some cities,  and bussiness ran sox lighting in those days.   Problem was lamps were expensive to replace so lot  of them got replace  in the 1990s for cheaper HPS.  Then there places that kept them like observertory,  I know a public storage unit had 35 watt sox wall pack, until they could not get the lamps any more. 

It was similar in Canada too, there was a push to have LPS installed during the 1970s and 1980s with even hardware stores selling 18w SOX wallpacks and 55w SOX streetlights at one point. Even some freeways here had the huge 180w SOX lights at one point.

My evidence for low pressure sodium lighting being so popular is that there were many fixture designs for low pressure sodium lamps for the UK market and there were several types of low pressure sodium lamps for the UK market aside from SOX. Those lamps were SLI/H, SO/H, SOI/H. In the US, we had fewer lamp designs for low pressure sodium lamps. They were the NA type lamps that had been used for street lighting and the imported SOX lamps that came from the UK.

There was indeed a wide variety, but in reality it was what funkybulb said, the 1970 energy crisis as I understand it

What's toxic in a SOX lamp..? Sodium possibly, but unless you go around eating it I don't see a problem.

Have you ever driven under LPS lighting..? Far better than anything else I've ever seen on the road. Colour rendering is not important; any of that you need is provided by your headlamps. Size of the lamp - yes they're large but as a consequence they don't appear as glare bombs the way smaller lamps do.

Barium at the electrodes to start with, getter mixtures,... All these tend to release once the lamp breaks in any way. Yes, they are present in pretty all discharges, but all spent discharges are dangerous waste due to these materials being used there, it is by far not only the mercury...

And yes, Ive been driving under SOX (from Brussels to Oudenaarde; pretty much all Be wa lit by SOX at that time), at night. Not during any fog nor heavy rain, but only at clear night and my impression was when compared to HPS around where I live normally, to me the HPS do better job. But I can not compare how it is e.g. during fog, even don't know if fogs are in some part of year a common thing around Brussel, where I live really dense fogs are pretty rare and both HPS and white LEDs seems to perform well, regarding visibility (definitely not worse than similar fog at daylight).

And to the glare bomb issue: If the lantern has a decent optics, it isnt going to be any glare bomb at all. To the contrary, the SOX Ive met in Belgium (pretty much all of them) seemed to me as way worse glare bombs than what I'm used to from HPS, MH and mosly many LEDs at home.
However many LEDs are glare bombs here, along with few HID types (mainly old MVs), but these I consider as s***ty or heavily outdated (mainly case of the old MV's) lantern optic designs, not fault of the light technology itself.
Is you are 100's m away, it does not matter anymore if the light source is 3cm or a meter long. Once it radiates large power horizonrally, it becomes a glare bomb. Or that is, how I percieved it. But again, maybe the really heavy fogs may change the game, but I havent encountered any, so can not judge.

Definitely SOX had their place in the past, when the accurate optics was not feasible. But in the 80's and 90's the optics around the HPS evolved so a 70W HPS was able to provide way better street illumination than a 90W SOX (side-by-side comparison in Oudenaarde, after apparent partial street upgrade)

What's toxic in a SOX lamp..? Sodium possibly, but unless you go around eating it I don't see a problem.

The sodium in the lamp and its products like sodium hydroxide aren't really poisonous, but corrosive, so even if you try to eat it you will only get physical damage. If you dump a bunch of SOX their sodium will become sodium hydroxide, then it will catch CO2 from the air and it will end as sodium carbonate, which the worst it can do is to make the floor where it's dumped a bit alkaline.

I think that SOX lamps are the "greenest" after incandescents.