The domain industry is essentially a storefront industry. As in, the product that provider A sells is indentical to the product provider B sells - if it's the same domain. The rules are set by ICANN.
I would:
- make sure you go with someone who has direct ICANN accreditation, rather than being a reseller of Tucows/Enom/etc/etc.
- make sure their frontend has good UI and isn't buggy. Not that there's a direct correlation - but it'll give a good idea of how well their systems work in the backend / how likely they are to send through your requests to the registry correctly!
- Make sure there's no too good to be true deal. Any domain provider that sells you a domain for £1, is likely going to charge more and be making all that loss back and more on year 2 - and 3 - and 4.
- Use a decent DNS provider for your DNS. It's hard to fault Cloudflare.
- Use a decent email provider for your email. Again, hard to fault google apps at ~£5/mo
This might sound like sacrilege as I used to run a UK domain registrar, dns provider and email host, but I now use Google Domains for my registrations, Cloudflare for my DNS, and Google Apps for email!