Am I the only one that uses British Airways and KLM?
During the last 14 years I've tried the others mentioned, and really don't think much of them - not as cheap or convenient as they are supposed to be, in my opinion.
As an example, I recently booked BA from Sofia to Newcastle one way. Cost £100 which includes 23 kg of hold luggage. Leave Sofia terminal 2 at 2.30 pm, arrive in Newcastle at 7.30 pm. One change at Heathrow (only 2 hours between flights) and luggage booked all the way through so only hand luggage to deal with on the journey. No long drives (and fuel costs) or train or bus journeys (and fares)
Similar journey time and luggage allowance with KLM, except the flight leaves Sofia at 7.00 am and the flight change is in Amsterdam and I am in Newcastle about 1.30 pm
The only other difference is that KLM work in partnership with Bulgaria Air so it's actually Bulgaria Air from Sofia to Amsterdam although the booking is through KLM.
Food and drink are provided on KLM but BA don't now provide free refreshments but sell sandwiches and drinks
There are, of course, flights to different parts of the UK from both Heathrow and Schipol (Amsterdam) - you're not restricted to Newcastle!