It's registered with a UK registrar - not with Nominet itself. I realise things can be set to "auto renew", but I never use that feature - I always manually renew my domains well before they expire.
Auto renew won't work if your credit card details have changed without you realising - and sometimes it fails for who knows what reason - valuable domains are far too important to trust to auto renew.
As for Nominet contacting me if it expires - well, they might do - but I don't want to count on it.
The 'worst case scenario' I'm trying to hedge against is that I'm lying in hospital in a coma for 6 months - when I wake up, I'm going to be in a real bad mood to find my domain expired!
If that happened, I wouldn't be around to receive Nominets warning email.