That's what I've done (my flight is the day after yours). While the UK government's web page is crystal clear that it's anytime on any of the two day's preceding the flight*, every single press and travel article I've read says 48 hours. I also couldn't find out if the UK entry required video observation like the US does, so I opted for that as well.
I've also scheduled two tests. One 47 hours before my flight, one mid morning the day before my flight. They don't talk about it much, but these tests have a 1-2% false positive rate (their false-negative rate is practically zero though) and I've been bitten by false-positive before**. If the first test is good, I'll rescheduled the second test to serve as my return flight test.
DaveB
*
https://www.gov.uk/guidance/coronavirus-covid-19-testing-for-people-travelling-to-england#when-to-take-your-testYou must take the test in the 2 days before your service to England departs.
For example, if you travel directly to England on Friday, you could take a test any time on the Wednesday, Thursday or Friday. The test result must be available for boarding.
** I was due to fly out in August, but had a false positive (disproved by follow up PCR test, but by then it was too late to fly). Mine was one of the 2 million Ellume test kits they've now recalled. So if your test was bought from Ellume and more than a couple of weeks ago, you should check their website to see if yours is affected. If it's on the recall list, the app won't let you use that kit, but by the time you're at that stage, it's too late to make other arrangements. They were supposed to reach out to everyone affected, but they never reached out to me. As the kits were issued under FDA emergency authorization, they don't have to provide any compensation. They are providing replacement kits, but delivery time is two to three weeks (too late for me and my remaining Ellume kits as I only heard about the official recall Friday night)
Edit: It didn't like my anchored hyperlink. Repeatedly.
