It's a good idea to cross-reference any UK domain registrar with TRUST PILOT UK. Read the most recent comments about customer service. Also read the negative review comments and see if the registrar bothered to RESPOND, and if it was a helpful one. More often than not if the Trust Pilot reviews are 4.5 stars or more it's a good company to do business with. It's worth carrying out this research before you make a decision as it will save your time and money.
Another thing you can do before spending any of your money on domains is to sign up for a free account with multiple UK registrars and submit a SUPPORT TICKET. See how long they take to respond, and if their response was friendly and solved your problem. I would wait a MAXIMUM of 24 HOURS for a standard (means not urgent) support ticket but expect a reply within 2-3 hours. They should also have a LIVE CHAT during business hours which you can also test.