Your budget is a bit on the low side even for a bike that you regard as temporary. If you can push it up to nearer £500 then you'll get some much better bikes that might actually last you a long time.
I commute into London regularly from Epsom Downs, doing about 100 miles a week so my advice too is UK specific.
At your budget I'd say no to disc brakes. Good disc brakes are very good but at your price level you'll get just as much stopping power from V brakes or calipers. I would say avoid chunky tyres, although these are easy to change, as they will create a lot of drag and slow you down. My suggestion would be 700c wheels as the choice of tyres is huge at that size.
I'd say the main choice you need to make is between a road bike or a hybrid. When I was young hybrids didn't really exist so I've ridden road bikes all my life. Personally I find straight bars uncomfortable but I know others feel differently. Have a sit on both and see which you think will suit you better. You'll get more wind resistance on a hybrid which, at your intended mileage, may be a factor, but otherwise there isn't really a lot to choose between them.
If you can stretch to £500 you can get one of the entry level Boardman bikes, such as the SLR 8.6, which are seriously good value for money.
In addition to Halfords and Decathlon you'll also probably find an Evans store near you and possibly a Cycle Surgery as well, so have a good shop around - you never know where you might come across a good deal. Does your company participate in the cycle-to-work scheme? You can get a bike through that and the tax man gives you a nice discount.
You'll find mudguards and the like surprisingly expensive so rather than let that eat in to your budget I'd leave it for now and decide what you want to do about it when the weather starts to deteriorate in a few months time.
As to 'bikes always get stolen in the UK', I've been a cyclist here since about 1975 and never lost a bike yet. If you have to leave your bike outside then get a decent lock, proportionate to the cost of the bike, and you should be ok.
Best of luck.
John