MTB wheels are smaller than touring bike wheels, which also means they're better for potholes and bumpy trails, but a wee bit slower on smooth tarmac.
700 x 35c tyres are reasonable for fine off-road use.
The specs on the Ridgeback Metro are all decent, but it is always best if you can try one first. You'd be surprised what suddenly bothers you when you get on the real bike. Besides, who is going to assemble it for you?
Almost any bike shop should bite your hand off for your business if you say, "I want to spend £250 (basic) and this is what I want for my money". If they make a good impression selling you the bike you'll also come back for service, accessories, the next bike, etc.
Bobike should work depending where cables go -- it looks like on the Ridgeback the rear brake cable will get in the way of fitting the Bobike bracket to the downtube (I think, can't find a good pic of the Bobike bracket that goes on downtube). If you went for a man's bike style instead, the rear brake cable would probably go along the crossbar, so not in the way.
The Ridgeback Motion LOOKS like the rear brake cable might thread thru inside the frame downtube, this would also solve the problem of where to put the Bobike bracket.
Why are you afraid of too many gears? 21 gears is not a lot of gears, believe me. It's only theoretical, anyway. Some of the combinations are impossible to use.
Is everything ok? You haven't said "hello" on the Feb08 thread for a while.