There is no evidence the CAA give a damn about people doing this. What they go after is overt charter work i.e. competing with AOC holders.
The problem is that a lot of illegal charter does go on, and the people that do it are quite clever in pushing the boundaries of the regs. I read about some really funny exploits a while ago, which the CAA tried to prosecute but failed, despite it being totally obvious "AOC" work. So we can expect the CAA to bang on about this at every opportunity - they cannot be seen not doing that.
If you are getting the passenger to pay 50% of the cost of the flight, the only thing you are in violation of is the "advertising within club premises" rule which would be pretty far fetched to draw attention.
However, the thing which would worry me is insurance not paying out, and this is serious. You could face a Graham Hill situation if you had a prang. Traditionally this would be dealt with by there being no agreement on payment until after the flight has been concluded without incident, but if you publicise it like you propose, up front, the paper trail does exist. I wouldn't do it.