Yes, Antigen is OK for re-entry to UK, and yes, you'll register with the provider and results are emailed. Should be sufficient to show on phone, but I would print out the email at your accommodation in Portugal.
Glad/Happy to help ...
FYI : Your times/days are similar to mine, but I'm leaving from Heathrow to Lisbon and driving up to Porto. I'm doing PCR Tuesday afternoon in Edinburgh(as that is where I'll be) and Rapid Antigen at Heathrow Friday morning before my flight later that afternoon. Then no hassle in Portugal, other than trying to make sure I'm in a fit state to get to the airport on Sunday :O