Where abouts in the UK are you? I can point you in the direction of several clubs that probably wouldn't mind you rocking up and taking a few measurements. Somewhere in the construction part of the forum are plans for an aluminium mast which might be do-able

I'm doing a LLF Mini myself and have picked up 3 windsurfing masts with a few sails for £30 which allowing for a bit of a mod to the bottom end will cost me about £20-25 each with all new materials inc. All aluminium will cost about £100. Blokart start at £170 for a mast for a 3.5m sail and up (x-sail could well be cheaper).
Cheers Col