A tough one, on the one hand you are away from home and running out of funds and need to return (perhaps for other reasons as well) and you're aware that there are issues travelling whilst ill. The airlines and the UK Govt are happy for you to travel and enter the UK. As soon as the word COVID is mentioned the mood changes and with the happenings since 2020 no surprise. The sub-variants of Omicron are now more or less akin to the common cold, even yesterday I heard of an 80yr old male who is covid +ve and feeling fine except for the usual annoyances of the cold virus. As the OP said, there most certainly are many people unknowingly cold/flu/covid +ve travelling at the moment, as it always was in the past (pre-2020). Yes it is inconsiderate to others, but at a certain point life has to carry on, as it has since time immemorial. Wear a well fitting FFP2/3 mask, be mindful to keep distance from others as much as possible and modify your normal behaviour to minimise risk to others.
The masses who have descended on the London area this last week seem to share one thing in common aside from their wish to mourn QE2, namely no masks, except in rare cases and we can be certain that some of those people will knowingly or not be afflicted with some form of communicable illness whatever it's type.