I am looking for suitable paint for restoration of some Fluorescent lanterns. They are made of sheet metal with White polyester enamel, which now starts flaking off and small touches of rust start appearing there

Normally i used White spray on enamel for appliance repair with fair results

This time, the luminaires are intended to be put to heavy duty work after the restoration (many hours/day), and i am not sure whether the spray enamel would hold up well to the UV without discoloring or cracking over time

What paint would you recommend for this restoration ?

Here in Canada, you can get high heat paint, I believe in both gloss and satin white. They are typically marketed, I think, to paint baseboard electric heaters. I'm not sure of the UV resistance, though.

Here in ‘Murica, you can get spray paint for automotive applications, as well as “high performance” professional grade enamel in both spray cans and standard paint. There’s also multiple grades of high temperature enamel paint, intended for painting car exhausts, engine parts, manifolds, etc. which I’ve seen on scamazon. Unfortunately, you would have to bake the parts to have the enamel paint fully cure.

There’s a lot of options for this; it all depends on how much money and time you want to spend. Personally, I would just try the “professional” enamel, if it started rusting I would then use the automotive or high temp. paint. Being that it’s a fluorescent fixture, I don’t believe you have to worry about heat problems for the paint, only weather resistance, so regular enamel for metal should be fine.  

If you’re really worried see if applying a clear coat would help.

If we weren't living under the shadow of Coronavirus I would recommend getting them stripped and powder coated, but I'm not sure if that type of work is being done just now.

Even when the powder coating is not available, I would recommend to first strip the old paint too. Just to be able to fix the starting corrosion. Otherwise it will rust in no time with whatever new paint on.

After the cleaning, I would go for auto paint "sandwitch" (from the base coat, till the top layer; really use all layers), because these are designed for quite some UV and mild heat exposure (normally coming from the sun).