Commuter bike for a 2x6mile ride in all UK weather with the capacity for some luggage:
You can use flat bar or drop bar, its up to you.
I really like cable disc brakes for UK winter use, they are very reliable and not affected by standing water or mud.
Boardman bikes are readily available, well designed and nicely made. The support from Halfords stores (ubiquitous car parts/bike store) is variable but you can get them from other shops. They do a disc brake hybrid and very nice disc cyclo cross both with rack and fender threaded eyelets.
You need to budget for accessories: lights, fenders (SKS chromolplastic), rack, luggage, lock.
Security is a much harder when you ride a flashy bike