Just so you don't think you're being ignored, I'll just point out that this might not be the most useful place to ask. There might be a few people with the right knowledge, but there aren't a heck of a lot of "collectors" here, nor do many of us focus on more recent material (the downloads available here date back to the '50s at the latest).
As with any collectables, your best bet to test prices is probably to check the relevant price guides (I don't know if there's any specialty material on British books) and compare them with the final auction prices on similar items out eBay way. Between the two, that should give you some idea of what people are willing to pay for particular books.