Airline question this time

I have tried to get the answer from the airline with no joy. I am leaving Portugal on the first leg of my journey on the 23rd of September to visit both children and grandchildren in the UK. My question is I have booked on with Iberia and will fly London to Barcelona and then Barcelona Sao Paulo, has anyone done similar? It's a connecting flight and not a stopover and I was wondering re documents if I show them leaving the UK and again when arriving in Brazil or will I also show them in Barcelona?  many thanks as always.
Arrival Barcelona:
Passengers who are fully vaccinated against Covid-19 must hold proof of either having received 2 doses of a vaccine approved by the European Medicines Agency (EMA) or the World Health Organization (WHO), or 1 dose of a vaccine not approved by these agencies, with a second dose of an approved vaccine at least for 14 days prior to travel. If more than 270 days have passed since the last dose, the certificate must also show that the passenger has received a booster dose.
Entry into Brazil:
Non-resident foreign travelers will need to be fully vaccinated to enter Brazil. Your last vaccine dose needs to be at least 14 days prior to your arrival in Brazil.
Hi thanks have all three shots of covid last one aroun 8 months ago Texan, it was more a question re all documents. I am full Irish but I have a UK passport, I also have a valid shengen permanent visa card, the question was more if I am airside on landing in Barceliona with a through ticket to Sao Paulo will they again ask to see my documents as many here are having real issues in the European Union with the British passport.
keep a close eye on conditions at Heathrow.....they have been cancelling fllights in and out like mad all summer.....
Thanks Inubia, I have been monitoring, thankfully its October so holiday rush will be over, It's up to them to get me here that or an other day. my daughter lives 20 mins from heathrow so if all fails i have accomodation. I have checked the flights and they are running at least on my planned route, early days mind.