I'm born & bred islington, & many years ago there was a great site at torrens St, side of the old angel station, cqb site called elektowerkz, long gone unfortunately, not many options now for central London non drivers.
One option you could consider now spring is almost sprung, some sites allow camping the night before a game, imperium at Faversham immediately springs to mind & I'm sure a few others do the same, might be an option if you want to hit some sites further afield, maybe rope in a mate or two make a jolly of it ?