Just a few points ..............
Ryanair & Easyjet are the best known low cost carriers, but there are many more.
Ryanair is the cheapest, has the least frills, and is highly succesful. It carries more passengers than BA, makes more profit, and has a higher stock market capitalisation.
They use a variety of airports. Some flights use main airports, other airports are a little distant (Beauvais), others are closer (Gothenborg).
Bookings are 99% via Internet. Prices vary with demand. Go "against the traffic" (say Spain to UK on Friday night), and book early to get the best prices.
Checked luggage limit 15kg, hand luggage (yes, they check it sometimes !) 10kg.
They quote a basic price, then add taxes, airport charges and a credit card fee. They offer options for desk check in, hold luggage, priority boarding, insurance, etc. Snacks and drinks are for sale - but not cheap.
I've just booked tickets for a trip Marseille-Madrid-Marseille. Flights in January, and mid week, of course !
Prices (both ways) are :
Flight : €0.01
taxes, charges : €9.99
One way cost : €10.00
Return cost : €20.00
Credit card fees, checked luggage and other charges came to €18.00, giving a total cost is €38.00 return. This is lower than the tolls on the autoroutes ( €58.00) if I was to drive, and cheaper than the car parking at Marseille. Of course, it is much cheaper than car parking in central Madrid !
Both Marseille and Madrid are the regular main airports.
In my experience, Ryanair always deliver (very) slightly more than they promise in their extensive list of terms and conditions.
So, think of a bus service, but cheaper, and you will be happy with Ryanair.