You can go for a cheap holiday to Alicante in a few hours from Luton. How long would that take by train? It’s quick and easy to get all around Europe by train if you live in, say, Germany. Simple geography rules that out for the UK.
Of course the journeys to southern Spain take too long to appeal the mainstream, and it is easier to purchase plane tickets, too.
To answer your question, it takes 12 hours to reach Barcelona from London and a few more the next day to reach Alicante…
My point was not that we should judge connectivity by distance to Alicante. My point was that, being an island, the routes from the UK to arbitrary cities in Europe are particularly long, especially if you are judging them as if they were a typical European country. If you prefer, swap out Alicante for Prague or Rome, or swap out Germany for France or Italy. The UK is isolated by the sea, so it should come as no great surprise that ground travel loses out to air travel.
> from London
You’re missing out part of the journey. I see multiple people getting tricked by the name of the airport in this thread. “London Luton Airport” is in Luton, not London. The start of the trip is not London, and you’ll need to make your way to and through London by train to compare fairly to London Luton Airport. Same goes for most of the other “London” airports.
Wouldn't German travelers also take advantage of French high speed trains when travelling to Spain?