Eurostar from London (Or Ashford/Ebbsfleet if you want to avoid London and park right outside the station) to Amsterdam on your first day.
1/2 Days in Amsterdam - There's an unlimited amount of things to do in Amsterdam, my favourite being sitting outside an (actual liquid) coffeeshop and watching the world and people go by.
Then Train to Brugge for 1/2 days - Possibly the most beautiful place i've ever been, it's like a ****** fairytale town. Go up the Belgry, drink lots of gay beer, eat lots of chocolate, go the Salvador Dali museum, take a boat tour on the canals, Pommes met Andoulouse all day err day.
Eurostar home from Brussels - Whether you spend a day here or not, I found it to be an absolute dump personally and would much rather spend the time in one of the hundred other beautiful towns or city in this part of the world.
Assuming your going for 6 days, this gives you a day either side to travel to and from the UK, you could always squeeze Ghent, Rotterdam or book your Eurostar home from Calais and stock up on French goodies from the big Carrefour at Cite Europ near the Calais Eurostar terminal
Eurostar is amazing compared to flying, just sitting in a nice comfy seat and watching the countryside go by is much more preferable to cramming into a Ryanair flight with one tiny bag