Harry - this may not be much help for you but check the used auto part listings on ww.craigslist.org.
Yeah, craig's list. The problem here is that the listings are for specific geographical locations, and most of the action is here in the States, not the UK. So, you'll be stabbing in the dark, with doing searches in different cities, states, etc.
I have seen - but not recently, mind you - some of the old style chrome racks for '60s and early '70s VW bugs and busses. The racks for bugs may work, if they are rear mount, not top. Forget the bus luggage racks.
If you find one, you'll be able to get it for cheap, the big question is fit and trusting the seller to ship to the UK. Of course, the added expense won't be a problem given the exchange rate you get to work with. Generally I've found Craig's List sellers to be pretty trustworthy, given the site's organic origins in San Francisco, but scammers are everywhere.
I regularly check the Denver/Boulder, Colorado listings, so if I see anything, I'll give you a holler.
I guess if you get bored at work, searching Craig's List can pass the time, but it's a bit like shooting at skunks in the dark. You may hit one, but it could stink!
Good luck.
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Tom