It is a neverending source of wonder to me that, whilst our government may be a shambles[0], our government services are so fantastic. They really are very impressive and, for the most part, very easy to use. Clearly a group of people have invested a lot of time and effort into this, and to me they demonstrate very clearly the value of investing heavily in user experience.
Also, the site is almost never down, which is good for somebody like me who tends to remember that they need to do thing X at random-o-clock at night.
[0] Not intended as an especially partisan statement as I have been known to vote Conservative (and over the last few years have criticised both Labour and Conservatives pretty equally), but I think it's tricky to argue that, as an example, sending 3 million kids back to school for 1 day before changing your mind demonstrates competent leadership.