Accessories are typically cheaper online - but if you buy a bike from an LBS, they'll usually give some discount on accessories and fit them for you. Clothes and especially shoes and helmets, better to look in shops -
Decathlon good for budget clothing,
Wiggle's own brand, dhb, is very good vfm.
The triban is often recommended to newbie roadies as it costs £300, and is far better than every new other bike at that price range - google for the Cycling Active review. Also, unlike
Halfords, the staff usually know how to put a bike together and tweak it / fix if it's not quite right
If you are happy to spend £600-1000 you will have far more choice and that's where the LBS comes into its own as opposed to the chains / sheds
Buy a bike that fits you, that is at a price you are comfortable with (if you don't get on with cycling are you prepared to lose 30-50% in depreciation when you sell it next winter) and that you like the look of - you are much more likely to ride it.
I would budget another £100-200 for accessories - lock, helmet, lights, clothing, mudguards etc etc