Looks like security staff at LHR are due to go on strike from 24th June and again every weekend until the end of August. T3 and T5 in particular are being targeted. It might be worth allowing extra time if anybody is planning to fly from either terminal on a weekend this summer: -
https://www.bbc.co.uk/news/business-65831998
https://www.bbc.co.uk/news/business-65831998