I have seen Jannet. She is in her late 30s I would estimate but she has very large natural breasts, a round bottom and pretty decent service. I wouldn't return, though, because £250 is too steep. Busty Juliette has retired unfortunately for you because she had better breasts than Linzi Dawn McKenzie. Asta, who features for a few agencies is VERY naturally busty too. She's older, like Jannet.
If I had £500 then I'd take a chance on the girl in the OP. You can always walk if she doesn't live up to the photos. She might be worth it and the huge price might simply be because she has 1 in 10,000 breasts. Sometimes this is the case.