if a vehicles drive wheel is on the road it has to be taxed

Is it me or would a bike rack not be twice as good? To me that's asking for trouble espcially with the dopy gets driving around now.

LOL, I don't believe that any current legislation forbids this in the UK per-se, apart from legibility of car rear lights and registration plate.

You may get away with car lights being partially obstructed (when turning), however - the reg plate will be obstructed at all times from the rear,

If the bike had a light board on it's rear, or the car on it's roof - then this would be legal (absolutely nothing to say that it isn't).

I wouldn't use this personally, with minimum round-trips at 100 miles to trials sites - I would't want my rear tyre worn out by tarmac!

With a bike carrier at around £100 delivered - why commit yourself to endless tyre replacements?

Red ;-)