Specialised Sirrus is a good bike, or consider a Trek 7.1, maybe 7.3. (I would say that, I have a 7.3).
Don't disregard buying used, you could get a very nice one for £200 or less, that leaves you up to £150 to upgrade with mudguards, a rack, lights, and maybe a pannier. You will need a lock too. Don't scrimp on locks.
Extras I know, but if you are commuting, you want clean and reasonably dry legs and a clean dry arse, hence the mudguards. The rack and pannier is good if you carry clothes into work. Saves you cramming everything into a rucksack, you'll get a sweaty back. Lights are a no brainer, at this time of year you want good LEDs, so save maybe £40 to get a good set.