The quarantine rules are based on having spent time in a given country in the 14 days before arriving in the UK, regardless of the mode of transport.
The only time when mode of transport becomes relevant is:
- you are travelling from country A, which is deemed 'safe' by the UK (i.e.: on a 'travel corridor'); but
- you travel via country B, which is not deemed 'safe' by the UK.
In that case, the manner of your transit via country B becomes relevant. If your transport through country B does not involve any stopping or mingling with the local population, then you may not have to quarantine.
At present, the Netherlands (your "country A") is not deemed 'safe' by the UK, so you would have to quarantine regardless of mode of transport.
Example -- Germany (not subject to quarantine)
However, if your "country A" had been Germany, which the UK currently deems a 'safe' country, then mode of transport becomes relevant, as the following examples show:
- Germany to the UK via train -- unfortunately, there are no direct non-stop trains from Germany to the UK, so you would have to quarantine (because you have to transit in Brussels, and the Eurostar would probably stop at Lille -- that gives two transit countries (Belgium and France), which are both deemed 'unsafe');
- Germany to the UK via direct flight -- you would not have to quarantine;
- Germany to the UK via car and ferry -- there are no direct ferries from Germany, so you would need to drive to the Netherlands or France (both 'unsafe'), and, even if you never leave your car while transiting those countries, you cannot stay in your car on the ferry, so you would mingle with ferry passengers who had just been in an 'unsafe' country'
- Germany to the UK via car and Eurotunnel service -- if you never leave your car while driving through France/Belgium/Netherlands (not even to get petrol), and you stay in your car during the Eurotunnel trip, you would not have to quarantine.
If the Netherlands becomes a 'safe' country again, but France and Belgium do not
Let us imagine, hypothetically, that the Netherlands is deemed 'safe' by the UK, but France and Belgium are not deemed safe. Then, you would avoid quarantine in the UK (assuming you had not been to any 'unsafe' country in the last 14 days) if you:
- got a direct ferry from the Netherlands to the UK (there are at least three options);
- got a direct flight from the Netherlands to the UK;
- drive a car to Calais (in France) without exiting your car while in France and Belgium (and without taking any hitch-hikers from those countries!) (ensure you have enough petrol!) and then getting the Eurotunnel service to the UK, still without exiting your car.
- [MAYBE] the direct Eurostar service from Amsterdam to London due to start in October 2020 may be a possibility, depending on whether there is a risk of mingling with passengers joining the train at Brussels (I think the Brussels-London passengers would be in a separate portion of the train, so it may not matter, so long as nobody can walk between the separate portions).