I had to deal with public administration in the UK, Italy, Spain and Sweden. The UK one is probably the easiest to use. You can definitely see that there is an excellent team behind.
There is one weakness, in my opinion, but it's not the service's fault as much as a national problem: identification is still messy. Whereas most continental administration have standardized on some sort of electronic-signature standard (which can be used to communicate safely and reliably as well as signing actual documents), the UK has not. The result is that the ID system is still excessively amenable to fraud and…
The flip side is that if you don't want this ID it becomes more difficult (but not impossible) to access any digital service. It also relies on an integrated public population register, which some people have problems with. But it's not a physical ID that you can be asked for or are required to have, so doesn't have the ideological & political issues of requiring a universal ID card.