Learned ones, I remember that, for super-quick visits abroad (I have one such thing, out Thursday and back Friday morning, all in all 28hr outside of Blighty) I can use the same test for both departure and arrival in the UK. In other words, I can go on Wednesday evening to ExpressTest, have my LFD, get the results, fly out Thursday, do the needful, come back Friday. Local requirements are for test + vaccination and I'm good.
However, my local hosts aren't sure it's correct and would like to see the chapter & verse of the UK Legislation where this is allowed. I tried to check the SI's, gave up and went on to read ESA's quality requirements as it's actually easier to understand. Can anyone help? Thanks in advance.