I would certainly expect the CAA to be keeping a close eye on it and possibly sending out some 'mystery shoppers' with tempting offers so if you value your licence then be careful.
I'm not sure the CAA would bother to do this but, if they did and caught someone breaking the rules, they should take action against them in exactly the same way as they would in the case of any other offence.
The essence of this is that cost sharing is allowed. The added dimension provided by Wingly, Skyuber, and indeed the Spare Seats thread on here, is just the mechanism by which those rides and their costs are shared.
Given that those mechanisms seem to be acceptable to the CAA and EASA (but not
AFAIK to the FAA!) and that pilots have insurance covering injuries to passengers, whether cost sharing or not, I don't see anything unreasonable in people doing this so long as they follow the rules.