Enforcement of face coverings on public transport
You are not allowed to get on public transport if you are not wearing a face covering, unless you have a legitimate reason for not wearing one. Transport staff may tell you not to board or ask you to get off.
If you refuse to wear a face covering, you can receive a fine from the police or Transport for London enforcement officers. The fixed penalty notice will require you to pay £100 (reduced to £50 if paid within 14 days).

When you can remove your face covering
You should remove your face covering if asked to do so by a police officer or other relevant person.
It is important to wash or sanitise your hands before and after touching your face covering. For longer journeys, take more than one face covering and a plastic bag for used face coverings.
You do not need to wear a face covering if you have a good reason not to (this is known as a ‘reasonable excuse’ in the regulations). This includes:
not being able to put on, wear or remove a face covering because of a physical or mental illness or impairment, or disability
if putting on, wearing or removing a face covering will cause you severe distress
if you are travelling with or providing assistance to someone who relies on lip reading to communicate
to avoid harm or injury, or the risk of harm or injury, to yourself or others
to avoid injury, or to escape a risk of harm, and you do not have a face covering with you
to eat or drink, but only if you need to
to take medication
if a police officer or other official requests you remove your face covering

It certainly makes a mockery of the 'Face Recognition Technology' that I am so against. With everyone wearing a mask, no one has a clue who you are any more... Great stuff... hate the thought that cameras follow you everywhere and invade your privacy.