LON - AMS 3.40hrs. in relaxed comfort. I'd take it every time. Car + ferry is equally relaxing, once you've got past Antwerp & Brussels in either direction. Sunday, no traffic, its 3.5hrs to the ferry, 2 hours on the other side and 90-120mins in-between, plus customs etc. i.e. all day. Weekday, though, it is hell and needs long buffer times not to miss the ferry.
Transport to a London airport, time plus cost, variable, but not quick nor cheap? 1.00hr block to block, 30 mins bag pick up + passports, transport to city centre 30 mins.
Relaxed friendly train travel, less crowded station compared to horrible airports (at one end), sight seeing or working time en-route, no nit picking staff etc. Why wouldn't you choose that? I don't understand why passports are not checked at departure; why in Brussels? Belgium & Netherlands are Schengen. If there can be passport checks at the ferry docks, why not at the train station? Changing trains and being passport checked is not brilliant, en-route. Is it the reason you have to disembark with all your luggage? That would be daft.
I admit I habe not made the current train journey, only flight or car/ferry. I'd welcome any knowledgeable pointers on the train option.