I can't recommend a bike I'm afraid, but I will say this: avoid Halfords
I decided to get in to cycling a couple of summers ago and made the mistake of buying a cheapo bike from Halfords. Ended up spending almost as much on it replacing the crappy gears, tyres, pedals, chain and saddle; it's almost a different bike now! Their service was crap too (bike and customer), ended up watching hours of YouTube and learning to service the bike myself.
It's OK now, not great, but for the time and money I have put in to it, I'd have been better off just buying a decent bike from the start!
I know Halfords have other, reputable brands, but all bikes need a decent service and you won't get that in [most] Halfords IME
I'd rather go online than buy at halfords. Something like this - Ridgeback Motion 2013 Hybrid Bike | Evans Cycles. If you want mud guards and a back rack I'd wait until Aldi get them back in. Cost you about an extra £20.
I'd recommend Edinburgh Bike Co-op stuff, well made & good value, they know what they're doing in there. Same for Evans Cycles too bit more expensive though. Try before you buy if at all possible, riding positions can vary a lot & get help in adjusting the riding position to suit you.
All my bikes have been second hand for example my latest bike is a cube arial ready for race 2009 RRP 700ish got it off a mate for 200 as he has moved to AUS. So I would recommend looking on ebay/asking around..also if you have a friend that is well up on bikes take them with you the money you save getting a second hand bike you could put towards a decent service at your local bike shop, its worth reading up/videoing up on general bike servicing skills plenty out there on Youtube.
I got one of these 3 years ago.
I've used it sporadically but I'm heavy and the wheels are still true and it all works properly. Replaced tyres and brake blocks once but otherwise it's still the same.
I used to race many (many) years ago and this costs less than one of my wheels from then!
How about something a little different ....
I bought the cheapest decathalon mountain bike I could then bought some new anti puncture commuting tires for it.
Total cost was about 150 quid. Runs really well.
Our county council offers employees an interest free loan to buy a bike payable over 12 months so I bought myself an electric bike for £800, love it cycling to work along the coast road, unfortunately some serious health issues have stopped me for now but i'll be back.....
Is that 'Cycle to work' scheme still running? That's how I got my bike a few years back. I think it allows you to buy a bike with the VAT shaved off the price AND you pay it back in interest free monthly instalments.
Type 'cyclescheme' into your favourite search engine & you should be able to find out about it. It might enable you to increase your budget.
I don't know nothing about bikes, ended up getting a Cannondale hybrid. It's done good for me and is really light.
I don't drive and bus fare here would cost me £4 a day so the bike saves me plenty £££££. My cycle route is 90% off road and it's GREAT!!! It's fun in winter and just ... relaxing in summer. Nothing like starting the day in a good mood.