Nittinghill,
I don't know about the book landscape in the UK, but here is how I would go about it from the states (I too, have thought about this venture but had to withdraw because of high shipping costs

) What I do now is get a list of books from actual people I know will be buying such books, get them to drop a deposit with a friend in naija (that way, I don't bring them over, only to be told they don't want the book anymore), then bring the books during my next visit. Due to extra luggage charges, I limit my freight to a certain number of books, but I will say though, that at least for me, the extra money does come in handy

Of course, I incorporate the cost of extra luggage into the cost of such books. Of course, yes I know, this is only on a small scale, but hey, it works for me! he he.
In the states, there are many bookshops that sell used books at 50% discount (minimum) as well as bookshops that are going out of business (thanks to the likes of borders and barnes and noble). For the former, you just check to see if what they have is what people are looking for back home. For the latter, it make b tedious scouring the country for such stores (which don't tend to advertise in order to keep costs low, etc) but the effort is worth it. You could buy a book for the equivalent of N50 and sell it for 3 or 4 times that, depending on what kind of book it is. You could even sell the book at the cover price and offer a "discount". When people want a book, trust me, within limits, they are willing to drop the dough for it.
Either way, just make sure that whatever books you want to sell, the demand for it is there. Finally, make sure that if and when you do set up shop, you do so big time and make sure you are always selling the most up-to-date books. Why? because from my experience, once Eze in aba gets a hold of it, he will re-print such books and sell them at a quarter of what you are selling for
Case in point: before my wife came to the states, there was a book I sent to her on marriage. Once she got it, she called me to let me know that she had the same book. I said it was impossible as that edidtion of the book had just come out. Once she did land in the states, I obtained the book from her only to find out that it was paper back, in black and white and "written and published" by a publishing press in aba

No fear of repurcussion on the re-printed book whatsoever!!
Good luck!!