I would take some screen shots of TAP and easyJet's timings today just in case, however this doesn't look like something in EC261 territory, it's not that late. I'm not sure if the problem is at the LHR end, or it's high temperatures. But the dispatch codes are ZY (rotational) on 519, which will be correct; whereas 518 is showing both ZO and ZY. ZY there may mean the crew or aircraft were held up earlier in the day. I notice that the aircraft previously came in from Dyce, but it was more or less on schedule to and from ABZ so BA must have allocated that aircraft knowing that this would result in a delay - it got to T5 around 12:10 and LIS was supposed to depart 12:50, so that was hopeless even assuming super slick ground handling. So my guess is that the original aircraft got swapped - perhaps due to weather issues and ABZ was the best of a bad hand.