If you use public transport in the UK, you must wear a face covering. Public transport includes trains, buses, trams, coaches, ferries, underground, planes, cable cars. In England, gov.uk has published the following information:

Travel safely during the coronavirus outbreak
Social distancing (and social contact)
See What do ‘lockdown’, ‘tiers’ and ‘protection levels’ mean? for information about restrictions on movement in your area.

Full details about travel in the UK are in government guidance issued by the four UK countries:

England gov.uk/coronavirus
Scotland gov.scot/coronavirus-covid-19
Wales gov.wales/coronavirus
Northern Ireland nidirect.gov.uk/campaigns/coronavirus-covid-19