As bizzare requests go this one is quite high up there. Perhaps I am too old to understand the whole "prom" thing...
Anyone being paid to drive henry's vehicle (or their own) will be doing so 'for hire or reward' and subject to a whole heap of issues to consider. I'm not actually sure that it is illegal to carry passengers in any trailer on the road (s90 of the C&U regs implies it is if for hire and reward - so be extrapolation it is not if unpaid?) - however other parts (e.g. s100) make clear that anything dangerous is dodgy!
Also be aware that some road traffic law applies anywhere the public have access to - not just to public roads.
Anyone wanting to help henry in his crazy endeavours might want to take a look at this - whilst not directly applicable it might give some food for thought about preventing a party turning into a new rib net 'tragedy' thread"