Work Permit System
Workers do not need a visa to work in the UK if they are from the EU, Switzerland, Norway, Iceland or Liechtenstein and they came to the UK by 31 December 2020. If a worker started living in the UK by that date they can apply to the EU Settlement Scheme for settled or pre-settled status, in order to continue to work, live or study in the UK: https://www.gov.uk/settled-status-eu-citizens-families . Workers must apply to the EU Settlement Scheme by 30 June 2021.
Workers do not need to apply for a visa if they are Irish citizens.
If an employer wishes to employ someone from outside the UK and they are not covered by the above, then the new points-based immigration system applies. Details can be found here: https://www.gov.uk/guidance/new-immigration-system-what-you-need-to-know . Under this system, anyone coming to the UK for work must meet a specific set of requirements for which they will score points. Visas are awarded to those that gain enough points. The intention is that the points-based system will provide flexible arrangements for UK employers to recruit skilled overseas workers through a number of different immigration routes.
In addition to the points-based system, employers will now need a sponsor licence to employ staff from outside the UK: https://www.gov.uk/uk-visa-sponsorship-employers . The licence is valid for four years.