Hi,
It is definately not illegal in the U.K. as you say but you have to be able to justify it if asked I believe.
They obviously would not expect you to let our snake starve to death if it would not accept F/T (and indeed would prosecute you if you did) but equally may decide to prosecute you for cruelty to the prey animal if your responce was "cos it looks cool to see it die" or something equally vacuous.
They are sensible, reasonable people for the most part who share our love of animals at the end of the day.
On the getting the feeders part it is illegal to sell a live mammal under six weeks of age from what I can find out so some sizes of prey item will be difficult to get unless you breed them yourself.
This may only apply to pet shops however and I am far from a legal expert on any of this.
dr del