While UK train prices are an outlier, London and the South East are very much part of the high density economic core of Europe. A box drawn around London/Paris/Amsterdam/Cologne contains roughly a quarter of the pre-Brexit EU population (and a substantially higher proportion of economic activity.)
Rail is already very competitive between these cities - it's only 2 hours on the train from London to Paris or Brussels and since the tunnel was built Eurostar has captured the vast bulk of travel between the cities it connects. The bigger challenge would seem to be making rail competitive to more dispersed cities like Berlin/Vienna/Barcelona/Madrid.
Getting to LHR from where I was, it was always quicker to get to any of the major rail stations in central London than to either LHR or LGW, but LCY was relatively good (only really works that way if you're east of the centre though).
You can get a coach (bus) direct to Heathrow from almost any town in England. Getting to Waterloo [1] (London’s international) station from outside London could easily be more time consuming and tough with luggage.
But of course UK is gonna be an outlier in European train value discussions. It’s an island on the periphery with a single rail connection. The mainland capitals are more competitive.
[1] edit: now St. Pancras, which is closer to other London rail stations so likely better on balance. Thanks user sbuk.