Simply head to any area with a wide ethnic mix, or where there are plenty of East European migrants, and look on the doors of newsagents and convenience stores run by Asian families or (for example) stores selling Polish foods. The vast majority of such shops seem to have posters, advertising that they sell Lebara sims, on their doors or in their windows.
That's what I wondered Buenchico, it's just for Mr N when we go there, I always have an Orange sim for England, and he so rarely goes, that I've seen they are remarkably cheap and I can bung it in an old unlocked phone of mine.