White Butterfly

I think it is a butterfly, but it might be a moth. I cannot find one on the Butterfly Conservation site or iSpot; hopefully someone will know what it is. Although it is not a particularly good picture, I have seen so few butterflies this year that it had to be blipped.
 
A day of doing very little although I have caught up with my temperature blanket.
 
The temperature was twenty degrees Celsius at GMT noon, with intermittent rain and sunshine.