I don't know about the UK, but in Ireland we are required to have our drivers licence with us when driving. So it's not practical to have it with your pilots licence.
I put my passport into my pilots licence, and keep them together. That why when I flew out of the country, I don't forget about it.
While I do not wish to loose my passport, I equally don't wish to loose my pilots licence, so it's not much of an issue security wise.
dp