It might surprise you to know that the rarest and most valuable book is actually On Her Majesty's Secret Service (followed by Octopussy). For whatever reason, the later books are very hard to find as true UK paperback firsts. The key is to find copies without a price on the cover. Check out page 122 of Bond Bound and you'll see that even their OHMSS -- which is captioned as a first edition -- has a price on the cover.
I should also point out that my You Only Live Twice, which is noted on the copyright page as a first edition, came with a wrap-around dust-jacket featuring the UK tie-in movie art and says on the back "Not for Sale in the UK", so I'm not sure what the story is with this edition. Also, my Live and Let Die is from 1957, but I've never seen an earlier edition so it's possible it was published out of order and this was the true first.
I'll take us into the 1980s and '90s next.