Damo666,
I'm not aware of any new, UNmodified, HF radios that will transmit on 11 meters using FM or any other mode. But then, I'm not familiar with any radios available to you in the UK/EU, only in the USA, sorry. That also goes for the legality of doing such a modification, you are on your own there.
The used radio market is a different story. Since there's no warranty, whatever you do, or has been done to a 'used' radio is just about whatever you can get away with. I don't know of a major brand that can't be converted, the only difference is in the difficulty of doing so. I would think that £700 limit would open up quite a selection of possible radios. From there, it's whatever strikes your fancy/fantasy?
Good luck.
- 'Doc