Groupon link: https://www.groupon.co.uk/deals/priority-pass-105

Priority Pass UK Membership types: https://www.prioritypass.com/en/join...BP&country=GBR
£6 for Groupon of 50% membership including first visit+50% of £69=£40.50 for 1 visit. For two visits (1 visit at each airport)= £55.50
If you're going to be traveling more often with Priority Pass Plus you get 10 visits included with the 25% off groupon rate it comes to £125.25.

Lounge Buddy Cost: @ AMM $39.00 ~ £30.28
Direct purchase: @ LHR £34.99