I'm not that well-versed on UK butterflies, but I'm going to say that this is a Green-veined White (Pieris napi). Do a google images search and see if you agree. It's sometimes hard from just one image and this doesn't show the forewings. But I'm going to stick with the Green Veined White
It's a very nice capture and you were lucky to get this close! Maybe the approaching fall has them a little more docile than typical. I may play with the contrast a little more. I have a feeling the actual butterfly can pop a little more. Just a thought.
But thank you for trying to stump me I love a challenge!!!
frankie