I had exactly that issue the other day - not only UKP but any "adult" site. I was away from my usual area at the time.
Previously, when this happened, it was down to my mobile carrier doing some kind of upgrade which reset the adult content filter to "on". Trying to check to reset this was a nightmare, could only be done via WiFi. Found a pub with free WiFi, and it wasn't the adult content filter.
Decided to go to my mobile providers shop. Parked up near the shops - back home in my own patch - and everything worked as normal. Odd.