For a one-off, Drake's Island - Nothing else like it in the UK... Although I'd also say Z-mart where I played over the years, very tight CQB with amazing staff and a big break from the more serious atmosphere of my local UCAP sites. Never had a bad day there.
I really miss Vanquish run by UCAP in Dover. Laid out in an out napoleonic war fort on the cliffs, was used as a prison and a film set so had tonnes of props and space all round. Such as shame it closed down