A friend of mine is building an airbike in the USA, looks like great fun. He managed to get a set of plans rather than buy the kit, so that makes it cheaper, but a longer project.
As said earlier, the original company is no more, after some nasty lawyer took all of their money.
However the Minimax line continued with a new compnay, so I wonder if the Airbike was taken on by someone else.
To answer your question of when they will be "legal" in the UK, that will be when you or someone else buys one and provides the PFA or BMAA with all the data needed for its approval, much the same way as any other homebuilt or microlight gets approved.
Go for it!..