how to grow a poincinia regia in the uk

Hi Smiffy and welcome to GoY, hopefully someone can tell you differently but so far as I know this tropical tree wont grow in the UK, even in the States it only grows in places like Florida. Given that it is a tree keeping it in a heated greenhouse, unless you have an extremely large one, isn't going to work.

Moongrowers right - this is a tropical tree, will only grow successfully outside in Zone 10 and above - we in Britain are mostly Zone 8, with a couple of areas in Zone 7. Keep it in the heated greenhouse/conservatory until it gets too big, at which point you will have to get rid of it. If you keep it in the conservatory over winter, you need to leave the door to the house open overnight to let some heat in during hours of dark cold, or have some heating on for it.