Neither the website of the Cuban Embassy in London nor that of the country's official tourism body here have any relevant information. The website of the Foreign, Commonwealth and Development Office (which is usually a reliable source of information) simply refers to 'a PCR test':
https://www.gov.uk/foreign-travel-advice/cuba/entry-requirements
Home PCR tests and in-store ones are exactly the same and probably processed by the same laboratories too. However in-store testing (such as that offered at some branches of Boots) gives you the chance to prove your identity, as you have to present your passport at the time of testing. (Your passport number is then recorded on the certificate). It might be best to go for such a test, as there then couldn't be any doubt as to the authenticity of your certificate.
Home tests typically cost around £59, whereas in-store tests at both Boots and Superdrug cost £79. It might be worth paying the extra twenty quid for peace of mind when you fly.