Chartering an aircraft will always be more expensive than the commercial option. There will be two deadlegs involved going from UK to Bordeaux.
So it depends on the standard of service you require on board. Other advantages of charter is you go depart when and from where you want. Quicker access to airport using an FBO rather than the main terminal.
This premium service comes at a price and don't expect it to be comparible with 20 economy seats with Ryanair.
Send me a private message if you need any more help.
brgds,
GP