Flying to AMS via LHR. Would like to meet a good friend now living in London. I can arrive at 8 am from JFK, depart to AMS at 5. Given customs, security lines, etc...how much time will I actually have?

1. Time through customs (or is there a transit shortcut)
2. Time to London
3. Time back to Heathrow
4. Time through security

Haven't seen her in a while and she is worth the trouble...