The thing with Bulgaria is that for years only Balkan really offered it with any success. For Summer '04, it seems that the bigger tour operators are now also offering it (the likes of Thomson, First Choice, Thomas Cook etc) - which will mean an increase in flights. I can't see a british airline offering VAR or BOJ from for example ABZ or HUY - but a based a/c should be OK from the likes of LGW or MAN. The reason is that Balkan are trying to do what the old Yugotours did before they went under and offer holiday flights from the smaller airports - while the bigger tour ops are offering flights from LGW/MAN etc. I suspect that you will find that the charter from the likes of ABZ and HUY are fully sold as Balkan Holiday with the bigger aiports having the likes of AMM or TCX operating the flights with each of the tour ops concerned having a part allocation on the flights.
The ski market is somewhat different though - with the biggies having offered Bulgaria for a while on UK aircraft (MON/BY/AMM/and even BA) to PDV and SOF.