Finding a decent flat is matter of luck or good preparation. Unfortunately, there are dodgy landlords, horrible flats and difficult neighbours. I listed couple advices for UK flat hunting:

Reserve time for flat hunting. Prepare to stay in B&B for a week or two instead of rushing to sign contract for the first flat you happen to visit.

Get a clear idea how much you can pay and what you should get with that. Some areas are cheaper but there are usually reasons for that. Remember to check the council tax for the flat.

Visit several flats. Do not worry about wasting agencies' time, they get paid for that. Ask questions but be prepared to meet totally clueless agents.

Check the background of the rental agency. Ask your colleagues and check Internet forums.

Consider private landlords instead of agencies. Generally, private landlords might offer lower rent.

Get everything written. Lease contract, meter readings, inventory list and receipt of the deposit. Take photos of the flat and make sure you capture existing damages.

Ask about current gas and electricity suppliers. A flat without previous utility bills should be avoided. Opening a new supply takes a lot of time and it isn't recommend for people with weak heart.

Older flats have single glazing and carpets everywhere - even in the bathroom. This means higher gas bill during colder months. Carpets can cause problems for allergic people.