I think it's perhaps useful if we all remind ourselves that several years ago Gatwick changed its pricing policy so that small aircraft ended up paying almost the same per aircraft (not per pax) as a 150 seat A320 / B737. The result was that Flybe with 78 seat Dash 8 aircraft saw the airport charge per pax becoming unsustainable when considered against what price pax were willing to pay for tickets. Flybe appealed to the CAA but lost.
The move by Gatwick was likely an attempt to force small aircraft out - the airport at the time had a policy of aiming to increase the average number of pax per aircraft movement.
I imagine Stansted would adopt a similiar pricing policy if anyone tried putting 19 seaters into STN over the summer period. Luton has recently begun making life more difficult for private jets in favour of A320 / B737 aircraft. Slots at Heathrow will be far too expensive for 19 seaters and City is closed for 24 hours at the weekend.
Unless someone can come up with a big chunk of Govt subsidy (like Dundee) or get major political support in Westminster, the only viable London airport for Anglesey is (as stated above) Southend.