London to Lisbon without flying?

As mentioned, the Seat 61 Portugal page has most of the options for travelling there by train. Booking in advance, you can get it to be fairly cheap, but not as cheap as many of the budget airlines go. However, one thing in favour of the train is that bookings generally open 90 days before travel. So, if the cheap flights have sold out (which often go on sale 1 year in advance), but it's just over 3 months until you want to travel, you could hold on a little bit and try to get the cheap advanced purchase tickets on the day they go on sale.

Another option is to take the ferry to Spain, then train, but that's coming in even higher, unless you can get an unusually cheap ferry deal (unlikely during peak season)

One other option is to take the bus. Eurolines, for example, run buses through to Lisbon. In their winter timetable, they have 3 buses a week to Lisbon, for ~£100 one way or ~£165 return. I expect it'll be a little more in summer, but that is less than the advanced purchase train tickets. However, it does mean spending 36 hours pretty much non-stop on buses...