I would not dear it. STN is one of the airports where I arrive 2,5 hours before a flight to get in line first with Ryanair to proceed through security and get to the gate. I seldomly have more than ~30 min left once I arrive at the gate (and that is with things going smoothly.
Key factor is the lenght in security - STN is a major LCC hub so a lot of inexperienced people - plus I find security to be tighter aka more time consuming than everywhere else.
Remember that you need to check baggages and everthing also.
If you take up the cost and the stress, why not continue the flight from LHR if possible?