The Government has just emailed me the current requirements as they now stand. Note that it is 2 days for the pre-arrival test and 48 hours for completion of the PLF, and that transit passengers must isolate until they leave.
From the horse's mouth:
Before you travel to England – fully vaccinated
Before you travel to England you must:
take a COVID-19 test – to be taken in the 2 days before you travel to England
book and pay for a COVID-19 PCR test – to be taken after you arrive in England
complete a passenger locator form – to be completed in the 48 hours before you arrive in England
Read more about taking a COVID-19 test before you travel to England.
If you’re in England for less than 2 days, you still need to book and pay for a day 2 COVID-19 test. You must quarantine until you receive a negative test result, or until you leave England, whichever is sooner.
When you arrive in England – fully vaccinated
After you arrive in England you must take a COVID-19 PCR test. This is the test that you booked before travel.
You can take the test any time after you arrive and before the end of day 2 at the latest. The day you arrive is day 0.
You must quarantine in your home or the place you are staying while you await your test result.
Test results
If your test result is negative, you can end your quarantine.
If your test result is positive, you must self-isolate for 10 full days. The day of the test is day 0.
If your test result is unclear, you must self-isolate for 10 full days. The day you took the test is day 0.
If your test result is unclear, you can choose to take another private test. If the result of that test is negative, you can stop self-isolating.
If your PCR test result is delayed, you must quarantine until you receive a negative test result or until day 14 after arrival, whichever is sooner.