Anything from Halfords will do. Hammerite requires less coats but I find engine paint gives a more natural/factory gloss appearance. Sadly engine paint often needs three coats to really build up the colour. To be honest even bog standard enamel or even automotive paint should work if you clean and prime. I don't think calipers get that hot, not enough to worry the paint. I'm planning on doing mine body colour using enamel paint. Hammerite is probably easiest option though as you don't need to prime.