You're assuming that the flight from GLA lands on time. Hopefully it will, but LHR operates close to capacity and delays are a fact of life. Delays of more than an hour aren't normal, but nor can they be discounted - they do sometimes happen.
What will it cost you if you miss the second flight? Your plan has way too much risk for my appetite.
I would either book under one ticket number (to get connection protection) or get to LHR much earlier.