In reply to James Malloch:
Another recommendation for a cyclocross considering what you are planning (think of it as an incredibly tough road bike that you can take off road) if you don't think they're up to what you are after have a look at the conditions they race them in. I took up commuting to Leeds back in May and have since commuted ~12 miles both ways almost daily on cheap cyclocross using a mixture of roads, bridleways and towpaths. I even took it on the Big Leeds Bike ride at the end of the summer and wound up doing 98miles that day (and easily kept up with the majority of the roadies).
I'd look at the Boardman CX, they get good reviews, I doubt you'll be disappointed. Invest in some commuter tyres as well as the off road tyres the bike comes with when you order the bike (I use Schwalbe Marathon Plus, decent mile munchers with a built in protection strip, after 3,000 miles I haven't had a single puncture on them) that way for poor conditions and off road you can stick on the knobbly tyres, but for your daily grind you'll have the choice of the thinner, faster commuter tyres (I never fit the knobblies BTW, just slip around loads in the mud
).
Its a shame you have to use Halfords (though Boardman are good bikes) PlanetX in Sheffield and Barnsley do lots of decent bikes at really good prices. The On-One cyclocross bikes are apparently brilliant bikes for the money. It may be worth looking at their website, you could well get more bike for your money (so may not need the cycle-to-work scheme if you are saving a few hundred quid over a Boardman equivalent) and they'll do finance so you can still pay monthly for your bike. I think the cycle-to-work scheme can blinker people sometimes and artificially limit their choices when you can get better value by just looking elsewhere.