Do you think I can run this fanless? I would also put in a 2.5 SATA drive.
Don't know. There are boards can be run fanless in this type of case but I've never tried this board.
Are there any other options that are more powerful and can still be run fanless (and are easily available in the UK)?
Depending on what you want to do exactly, the most powerful Silvermont CPUs might be faster. Probably not by much though, and they'll be more expensive while being slower at other tasks. Still, a low-power architecture would probably save you a little electricity in the long run.
You probably don't need the power anyway. There are cheaper, less powerful boards which would probably suit you fine and run cooler.