I live in Liverpool. No way I am getting Eurostar to anywhere in Europe unless I just fancy the experience and have the time. Same for huge swathes of the UK. I used Eurostar once purely so I got to use it. There are millions of people who will never use Eurostar because it makes no sense and they aren't interested in trains.
Of course. Interestingly, just about half of the nearly 6m UK - Paris / Brussels
air passengers* travel to/from Heathrow, Gatwick and Luton. (The numbers from City and Stansted are negligible). In addition there’s about 9 million on Eurostar. Taken together, that means that only 20% of all the UK - Paris / Brussels air/rail market does not use London. So whilst there are large areas of the country where it doesn’t make sense to use Eurostar, in terms of numbers of people in the market it is a relatively small proportion.
Back on the subject of Amsterdam, it is a similar picture. 46% of UK - Amsterdam
air passengers travel via a London airport.
* some of this will be interlining at Heathrow or CDG and therefore not a true alternative to Eurostar.
Source: CAA route analysis 2017