I think...
1. You have to use the bike 50% of the time you use it for commuting to work on. However I'm not 100% sure that anybody does check.
2. If you're going to ride it on the road, get a road bike. You dont need a full suspension bike unless you're a committed off roader, remember you loose energy by compressing the suspension with each turn of the pedals so its harder work to ride ( important if you're not fit ) You dont have to get dropped bars look for a hybrid which has road bike wheels and running gear but flat bars like a mountain bike.
Personally unless you take the bike off road, I wouldnt even contemplate a MTB. If you must have a MTB get a hardtail and something with front forks that you can "lock off" so they dont move about when on road.
3. Halfords should be able to source you any bike you want, but if its not something the usually stock then it might take weeks or maybe a couple of months for the bike to arrive.
Dont forget to factor in a helmet and kit, some more expensive bikes dont come with pedals so you'll need to think about that to.
I might suggest a more specific cycling forum might be useful for research such as
www.bikeradar.com