Ferry to where? Hull from Zeebrugge? Bear in mind trains in the UK are brutally expensive, I checked from Hull to Glasgow (if you book in advance) is just shy of £70 return, a 5+ hour journey that can include up to 3 changes of platform. You might have better luck finding Ferry prices than me but they were pretty high too. On the other hand Ryanair do a direct 1 hour 40 flight from Brussels to Glasgow for just under £120 (as I checked, your rates will vary of course) though you you'll almost certainly need to add luggage to that (perhaps another £40 total). So it's likely to be comparable to Ferry + UK train in terms of price, but under 2 hours compared to 12+ hours land travel. Just something to consider. Depending on the timing it could add up to two days to the usable trip (if you want). Also, if you fly in you're unlikely to need the hostel on the very first night... you might want to wait till later in the trip.
Finally, as to the main question
where to go... why not have a look at the well-maintained Long Distance Walks section?
http://www.walkhighlands.co.uk/long-distance-routes.shtmlOne idea might be to skip the early, lower-land parts of the West Highland Way by taking a train to Tyndrum, continue to Fort William, after that continue on the Great Glen Way or East Highland Way.
There are lots of hostels, bunkhouses, and some affordable B&Bs along the way. Once you have a clearer idea of your route people can chip-in with local suggestions, else you can see where Youth Hostels are along the route you eventually decide upon.
https://www.syha.org.uk/where-to-stay.aspx