I don't think anyone in the UK makes a 5E3 kit, i haven't seen one yet.
Modulus amplification keep the chassis and transformers and sell them at £125, they also keep a turret board already staked, i haven't used their transformers so i don't know what they're like.
Bluebell Audio in Scotland sell the Hammond transformers, which i think are great, they have a website.
Caps, pots, resistors, valve sockets, Hotrox are reasonably priced for those.
To be honest a 5E3 is a great build, if you like i could make up a list of bits you would need to build one and some suggestions for suppliers. A layout is available from tons of places, Weber VST, Ceriatone, they all have good layouts.
What i would say about 5e3 layouts is don't put the 4 x 68k input grid resistors on the board, they're better at the input jack, i know they say mount them as close to the valve as possible but they really are better on the input jacks, it lowers the noise floor a bit and it's a damn site easier to wire.
Hope that helps a bit.