In reply to Longsufferingropeholder:
> You can take a bike on the Dover ferry can't you? Does that unlock any more options or not really?
You can take a bike on more or less all ferries. Not all take foot passengers due to the cost of providing terminal facilities and shuttle buses (though a lot do) but bikes just cycle over with the cars so they generally take them, usually at a very low cost. Just select "bicycle" as your vehicle when booking.
That's what I would do. Eurostar is irritatingly expensive and faffy with bikes.
Or am I reading that wrong and you don't want to go with a bike this time? If so and you don't want to use Eurostar then check out which ferries take foot passengers (some do) or use the Eurostar InterRail discount mentioned upthread. If using a ferry as a foot passenger build in lots of spare time as you will generally be first on and last off, it can be nearly an hour after arrival in some cases.
Or if you don't mind coaches, Flixbus international journeys (even from near home e.g. Manchester) can be dirt cheap at times.
Post edited at 10:11