Flybe have online check in, its called Q-buster or something like that, works in a very similar way to BA if I remember correctly, am not sure if you can change your allocated seat though.
You can also pick your seat at the time of booking at a cost of £5 per sector. I always thought that this was a bit cheeky, but now EZY & FR are charging you upto £7.50 for the priviledge of being one of the first on the aircraft (or the bus to the aircraft in some cases) this is actually really good value for money!