Last I checked LHR was not in the EU. You know Brexit and all that.... :-) . But to your point yes transit may be more complicated going forward. That is why I booked my flight later this week to Greece direct from US. In COVID times I place a much high premium on not transiting third-countries if possible. Now hopefully Greece will still be welcoming Americans come the end of the week. But if not most of what I have other than two intra-country flights is refundable so if there is no clarity by say Wed I'll just pull the plug to a staycation.