Loads of people say stuff like 'you can go on cheap ferry route from Belgium or something'

But I can't actually find a quote that's less than 400-500 gbp.

The key is to book early - avoid Fridays and week-ends and travel either very very early or late. Not easy if you have to book last minute. Might be cheaper to fly even?

I'd just fly to Manchester or Leeds and take a train onwards. Last time I flew with easy jet to Manchester and took the train to York; it was quite cheap. Remember to book your tickets (both flight and train) in advance and it should be reasonably cheap.

no idea where you are going to /from. If you have your own car, the cheap car ferry route is Dunkirk-Dover, particularly if you travel around mid-night, 2 or 4am with Norfolk lines, now DFDS, cost can be as low as £20 each way. The journey is 2hrs and is it a quiet and comfortable journey.
Driving through Belgium you avoid the toll fees of France.
But not sure if you will need to buy the vignette for Germany. Quick search on google said only for Trucks since 2005.
It is not the quickest way, depends how you like driving long distance and the time factor, good point you can carry a lot of luggage unlike airlines.
Be interested to know if there was a coach service covering this route?