I'd say either Glasgow or Bristol. Both have loads of culture and loads of things to do and see! I lived in Glasgow for 4 years as a student and was never bored - I've visited quite a few times since becoming a Responsible Adult and I still love it
The Thistle Hotel in the city centre is good - I think its on Renfrew St and can be booked through Lastminute
Bristol is also a fun city - again loads of things to do and see with great countryside around and loads of fab restaurants to pig out in
I recommend The Villager Restaurant in Westbury-on-Trym highly! The City Inn is quite a comfortable hotel - very contempoary and the rates are decent.
Oh, I forgot about Bath! Gorgeous, but much more expensive to stay in than Bristol or Glasgow. And it has more culture and restaurants than you could shake a stick at!! We stayed at the Bath Windsor hotel on Great Pultney St. This is one of the newest hotels in Bath and also one of the most expensive. Perfect for a romantic break.
(edited because I forgot to mention hotels!)