Also, as those who've read the
T5 security is a total disaster! thread over on the BA forum will know, for various reasons (incompetance? unofficial industrial action? not caring?) Heathrow is currently unable to deliver a fast+effective security experience with just 2 runways. Given how bad it is with 2 runways, if they added a 3rd and the additional flights that go with that, you're going to need to be turning up the day before to get through security in time...