Read that but if read further down on the same web page it seems to indicate if you take your test in the uk before you travel it has to be within 3 days, read the bit further down & let us know your views
Do you mean this paragraph?
It's another reference to days again, not hours, so it falls in line with the other guidance is how I would interpret it.
If you got tested in the UK on Thursday, then your 3 days to board a flight back to the UK using that same test result are Friday, Saturday and Sunday.
Your test day would be day zero, in the same way that your landing day back in the UK is day zero in regards to the day 2 PCR test you'll have to take.
In my example I land back from Porto on the Wednesday (day zero), so I have Thursday and Friday to take my pre-booked PCR test.
I can only assume that our Government are using days rather than hours to help eliminate some of the ambiguity (which is kinda lol cos it's still confusing as fuck even to someone like me who works in travel)
72 hours is 3 days and 3 days is 72 hours, however when you involve travel across different time zones and things like departure times and arrival times, it makes a lot more sense to use the day measure than the hour measure.
To satisfy the Portuguese Government, they want the PCR test undertaken within 72 hours of your departure to their country, just in case anyone has got confused by anything above.