Have a look in the Coronavirus Forum, where the UK arrivals thread has a ton of information and real world experiences. Short answer: antigen, LAMP, PCR tests are all OK. Antigen is a quick and less accurate version of PCR, and is OK, so long as certain criteria are met, notably a test certificate from the supplier. Even lateral flow tests if handled by a supplier in a particular way. If you get an antigen test from a reputable source, within 3 days of departure back to the UK then that is good enough. Border Force want some evidence, particularly on time lines, but less so on every detail of the paperwork. It's posible that NHS Lateral Flow devices will be allowed but it's not yet confirmed.

To keep this vaguely BA-ish as a topic. BA have some options listed out here too, with discounts itemised to those quoting BA's promotional codes.:
https://www.britishairways.com/en-gb.../covid19-tests