Once, many years ago, I visited the Porton ranges (work visit, I hasten to add), so I may technically have bagged a tile there. Though as it was long before Veloviewer (or even GPS) I'm sure it doesn't count.
Security has tightened since then. and on visiting Porton Down a couple of years ago I was shadowed so closely that there was no chance of any surreptitious tile-bagging whatsoever.