expensive
I do not live in the UK and where I live it is apparent that we hire aircraft somewhat cheaper than you do. But I do have an idea of the distances you are trying to fly and can give a hugely wide guestimate.
An aircraft that carries 20 people plus bags is going to be pretty big and pretty expensive too. Now i reckon you will be going somewhere like Glasgow or Edinborough. So flight is going to be about an hour each way. Such an aircraft is probably gonna cost you 3000 pounds an hour so i reckon 6000 pounds should cover it. That is probably a slight over estimate. Call some regional airlines to find out something more specific.
It will usually be cheaper to pay for your 20 seats on a commercial flight. But it WILL be cheaper to hire a bus. Whats wrong with road tripping i ask??