It is 3 days and I would expect BA out-stations to be aware of that, it's also the same for the USA so ground agents working for external companies shouldn't find that odd. More information in the UK Arrivals thread in the Coronavirus Forum. Having said that, since an antigen / Lateral Flow test is good enough - and they give results in under an hour - you probably don't want to push it to the last minute, just in case (e.g.) you miss your connection and get put on a slightly later service to the UK.