I've kicked about with the army for the last 30 years and am a Joint Service Mountain Leadership cert holder. I seem to have spent half my life in bags supplied by the lowest bidder!
A sleep mat is essential, and mated with a good down filled 3 season bag will see you OK above freezing almost anywhere you can get a bicycle in the UK. Don't touch a bag with a zip, they lose heat. A 3 season bag + some clothing for chilly weather, same bag minus clothes for summer. (get it properly cleaned every season or use a cotton liner).
Buy from a proper mountain equipment supplier, preferably in N Wales or Scotland from people that spend real time out using the kit. Avoid
Halfords, Argos,etc. Blacks not too bad if you must buy in a big town.
Previous advice about sizes spot on, mummy bag much the best design. Make them let you get into it to try for size ( I suppose you ought to take your shoes off first). Also ensure it comes with a good compression sack, a you will be surprised just how small a good down filled bag will pack down to.
Down bags will compress better, so more room for beer!
Be prepared to pay a little more to get real quality.
Toodle Pip