Monkeh,I can only suggest Hamfests & the like,though whether there are many in the wilds of Cornwall,I don't know!
Prices for 100MHz Tek,HP,& Philips analogs seem to be around $A130 at the one & a half Hamfests we have here in West Oz each year,which translates to roughly 85 quid in your money.
Reading through the various threads on this forum,they seem to be approximately that level in most countries,so you may be battling to get something of that standard for much cheaper.
Back in the day,the UK had a lot of Electronics instrument manufacturers,so you may pick up the odd Telequipment,Solartron,or something along those lines.
Those companies,& I believe,your Military,also used a fair amount of imported stuff,so keep on looking,even if you have to do a few road trips!