Entering would almost certainly be a No, if you asked a BA agent, in the absence of Settled Status, further complicated by the Home Office advice to airlines to simply accept passengers' word that they have Settled Status (no card is issued for this). Airlines have been told they will not be fined for EU passengers carrying ID cards only in this scenario. For airside transit, for EU citizens it's purely a matter of whether their documents are ok for their destination. If travelling from UK to Greece then the ID card is fine for example. Heathrow's Chief Immigration Officer can also authorise entry to the UK for people on expired passports, and in the case of an EU citizen with valid ID they almost certainly would do so even if it was a leisure flight to the UK. EU citizens have 180 days in the UK and non adherence isn't followed up in most cases. At BA stations they can call the CIO and often do. But they wouldn't in this case from places like JFK or ORD given the advice on Timatic. Somewhere like BNA or PDX they may end up calling.