The USA currently places the UK within 'Level 3' of their Covid travel advice, meaning that intending travellers should 'reconsider travel':
https://travel.state.gov/content/travel/en/traveladvisories/traveladvisories/united-kingdom-travel-advisory.html
The US is on the UK's amber list, meaning that people arriving from the USA are required to book and pay for Covid tests, to be taken 2 days and 8 days after arrival, before they depart. They also need to have a negative result from a Covid test, taken in the USA in the three days prior to departure. They have to quarantine in their UK accommodation for 10 days unless they pay for a private PCR test 5 days after their arrival. (If that test is negative, they then no longer need to quarantine but they still need to take the 8th day test as well).
It can end up being extremely expensive! I have friend living in Italy (which is also on the amber list an therefore subject to the same rules), who has a flat here in Suffolk. She was considering coming o her flat but decided that it just wasn't worth it, with each private PCR test costing £145 and with the Italian government requiring her to self-isolate on her return as well.