I purchased a brand new R4 compressor for less than $200. Four Seasons brand, it's been working well for two years. Came with a 2 year/24000 mile warranty.
Are there less expensive brand new R4's available to you in the UK besides the dealer?
If not maybe it would be worthwhile to import from the USA? I ordered from rock auto.
I'm happy with the air conditioning performance this hot humid summer here in NYC. It's adequate using R12 refrigerant.