First, check out if your employer runs a tax-free bike purchase scheme. These usually run through Halfords (or Evans) but Halfords can supply almost any brand of bike. Their assembly can be of variable quality depending on who works at the store. Local bike shops are generally more helpful.
For everyday UK commuting you need a bike which can take medium sized tyres, proper bolt-on luggage rack and mudguards front and rear. As a big guy you will need to pay attention to frame size in reach (top tube length) as well as height (seat-tube length) . You really should pick the brand based on who makes the largest size.

In the £3-400 range you are looking at a pretty standard hybrid and in truth threr is not a lot of difference in quality between brands. The Boardman range at halfords is excellent value for money and well thought out. I would use the Boardman Hybrid as a benchmark so if you pay more, do you get more?
Try and avoid suspension.
Disc brakes are one modern convenience that is useful, they keep working in the wet. The Boardman hybrid bike has simple cable discs that are good enough. Their only drawback is the difficulty of fixing the lugage rack to the threaded eyelets. Scott solve this issue in a neat manner by repositioning the brakes.


Most bikes come fairly bare so you need to budget for accessories and kit.
Lights, luggage rack and bags, lock, mudguards, pump, repair kit, helmet. Some shops offer a tyre upgrade to kevlar protected such as Schwalbe Marathon or the bomproof Marathon Plus. This is worthwhile, ensuring you reach work on time.
You can probably manage with any outdoor kit you currently have but it helps to have some comfortable winter kit.