dj6: there is also the issue of route licensing - now much more open than decades ago, and within the EU there is not really any issue any more for carriers based there. However, the UK is now outside those rules, we are a "third country". I'm not current on all the nuances of the issue for UK carriers, but I would be very surprised if seats on a charter flight operated between the UK and the EU can legally be sold to individuals as "seat only" travel.
I suggest you need to drop the idea of travel agents chartering flights - it's done by tour operators.