Belmarsh Jail was built on part of the site of the Woolwich Arsenal so i wouldn't mind betting the lags have a football team
It's still there, it just doesn't have any guns or ammo in it. I just find it ironic that so many major clubs have moved away from the area / thing that gave them their name, but United is the one that everyone focuses on.
But OT's postal address is still in Manchester (MUFC, Sir Matt Busby Way, Old Trafford, Manchester M16 0RA). When they changed the borders in the 70s, strictly speaking OT became part of the Borough of Trafford in the county of Greater Manchester. But United moved to OT about 70 years earlier.