As it stands, today, no you can't. England runs a Green, Amber, Red classification for arrival from overseas. At the moment, any country you have been to in the last 10 days has to be on the Green list to avoid some form of quarantine. USA is currently on the Amber list so if you leave the airport you will have to have a test and then isolate in a place of your choosing for 10 days. Also more tests required during that period. This is the UK government page that provides the rules and regulations... https://www.gov.uk/guidance/red-amber-and-green-list-rules-for-entering-england .

Will the rules have changed by the time of Spa (I've always wanted to see the Spa GP but haven't yet made it!), probably. Keep an eye on the web page for updated info.