Departing the country of residence for the country of citizenship has normally been given more of an "out" to travel than when the country of citizenship is the same as the country of residence. If that is no longer how it goes with departures from the UK to the US for US citizens with ILR status in the UK or UK citizenship, then that is news to me as I've not yet heard of US citizens being denied travel from the UK to the US under UK rules related to Covid-19 travel restrictions. But technically US citizens resident in the UK are subject to the "stay at home restrictions" in the UK and should not be visiting the US for "holidays"/vacation/tourism purposes as they are subject to the "stay at home" regulations in the UK.

If you as a US citizen were going to the US to have a meeting with a family lawyer for purposes related to the increasingly difficult situation your dad, you and his other loved ones are facing, I doubt that any UK law enforcement would be on you for that.