Used to use 123-reg as my go to. In time their dot co dot uk s became as expensive as a dot com.
I found namecheap. As a registrar they are significantly cheaper.
They are now my go to registrar for coms and uks.
I have loads of domain names my hosting space is elsewhere.
I cannot comment on the other Namecheap services, but their domain registration is top notch.
Edit- I accidentally allowed a domain name registered elsewhere to expire. I renewed it at namecheap at a price much lower than I had previously paid.
I have only noticed the negative reviews left by other people. I have been happy thus far.