Sorry that's not accurate.
Your day 2 test after landing back into the UK has to be a government approved one, and the reference added to your locator form before you fly. No requirement to buy an expensive test and take it with you, just research and make sure you can access one where you are... it's likely to be free or much cheaper in your destination.
Your pre-return lateral flow test can be done by anyone, mine was a free one done at the airport before I flew home.