Does anyone know or can make an educated guess at what the largest single building in Britain is or was? I know Templeborough Steel Works in Rotherham (now Magna) once held the distinction and Fort Dunlop in Birmingham has been mentioned but i need hard facts!
Don't know that actual answer to your question is but I do know that when Richard Branson wanted to check out the balloon for one of his Atlantic hot air balloon trips the only building big enough for it to be inflated in was the Cammel Laird shipyard in Birkenhead.
Open to interpretation. If you mean total area, the palace of Westminster is pretty big. In terms of total volume, Canary Wharf is probably a good bet. In terms of single interior volume, it will probably be a shipbuilding site (or possibly the Cardington Airship hangars). As an aside, for a few centuries, the tythe barn in Cholsey (Berks/Oxon) was the largest aisled building in the world! Not there now though, I'm afraid.
Using your example of Fort Dunlop. That building has around 335,000 square feet of floor area. Canary Wharf Tower at Canada Square (a single building), London has around 4 million. Not sure it's the largest though.