I do long distance road riding so here's my tuppence worth -
1. Road bikes are much better on roads than mountain bikes are. I've got both, but I hate riding the MTB on the road, it is way slower and therefore much less fun.
2. Road bikes are plenty robust enough for canal path use.
3. Any new bike at £100-£150 will just put you off cycling. Specially one with any kind of suspension. I'd recommend you spend £300 - £500, which would get you a basic tourer like a Raleigh Venture or a Dawes Horizon. Decent range of gears, strong, reliable and not too slow. If you want to spend less, go secondhand.
4. Go look at some bikes in bike shops to work out your size. You're far too big for a 56cm frame (I'm 5'11" and that's what I ride), you'll be somewhere between 58 and 63cm. Even if you're buying second-hand or via a cheap online store, try lots of bikes at your local bike shops. Places like
Evans are very expensive but have a good range of bikes to play with.
5.
Halfords are very bad shops (clueless staff and lots of cheap rubbish) but the Carrera and the Chris Boardman road bike ranges are pretty good value.
6. Drop handlbars are more comfortable that straight bars. I know this seems unlikely but the fact is that you get more hand positions so you can alter your position on the bike. On straight bars you have one position and that's it.
7. Lots of beginners think that smooth skinny tyres must have less grip than big fat knobbly tyres. True for mud and gravel, not true at all for normal tarmac. Also worth noting that the higher the pressure in your tyres, the less likely they are to puncture!