Peaceable Projects Inc. is a U.S.-based non-profit group that brings the vast resources of the wide world together with the ongoing needs of the people who live, work, and travel on the Camino de Santiago pilgrim trail network in Spain.
I did a quick search and found that TAP has a flight from Gatwick to Lisbon, prices starts at around 60-70 euro (one way). It depends a bit on the dates of course.
As for coming back from Santiago to London, you have the Ryanair flight from Santiago to Stansted, but also a ClickAir flight from la Coruña to Heathrow. I did book this ClickAir flight for a customer on Thursday for a flight tomorrow (Sunday) and it was €125,-. With earlier bookings you could get it cheaper I think.
By the way, there is a train to la Coruña from Santiago that takes about 1 hour and is about €4 - €5.