Try somewhere like the Cape Verde, it's usually hot over the winter/autumn months, and is cheap and accessible from the UK easily through package holidays (Thomson usually)
If not, try one of the Canary Islands.
Benidorm
I would pick somewhere on the Mediterrian, or if you can make it all the way to the USA, somewhere in Florida or California, like Disney.
Majorca, Spain. It is beautiful and really hot. And it has all inclusive package and it wasn't that expensive and the flights were from the U.K. hope this helped :-)