UKARA only applies to retailers, hence United Kingdom Airsoft Retailers Association. There is no penalty for a private seller selling to someone without it.
The act you're looking for is the VCRA. Feel free to Google it for all the Incredibly dull long winded legal mumbo jumbo that in reality is totally unenforceable because there's about 3 police officers left in the whole country, and 2 of them are busy dealing with someone who said something mean on Facebook.
I'm sure
@Rogerborg will be in here shortly to blow his legal load all over you.