I think i am right in saying that any conservatory that is separated from the original house by way of external doors is exempt from building regs.
However as soon as you take the doors off it becomes an extension which is then subject to building regs and more importantly the thermal standards part, which most conservatories don't get anywhere near meeting. This obviously needs to be looked at before buying or selling a house as technically the council could come and tell you to rip it all down as it's not approved.