I don't believe that it's an atomic clock - more like some kind of time master source. When the BBC left Bush House in 2012 they sold off a stack of this time equipment; presumably used in their radio studios. Maybe the famous beeps were managed by a Patek Philippe? It's one of my horological regrets that I didn't bid in the auctions at the time; if I remember correctly a lot of these went for something in the region of £1200 plus fees. They were in better cosmetic condition than this one and could have looked reasonable if mounted in a bit of custom walnut, for example.