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.