Connection time at Heathrow

Presuming this is all within the same terminal then under normal circumstances it would be fine with no dawdling (the minimum is 60 mins). Little margin against the first plane being late though.

Normally an 80 minute connection in T5 is fine, even if you land in T5C it is just go via the 2 minute underground to T5A, you are almost at flight connections, and then it is up to conformity and then BA security and on into T5A, where your Lisbon plane flies from. You are not entering the UK, if you checked bags they are checked through.

But--you are flying from Denver. And looking at recent history on Flightaware.com, your flight does not leave Denver timely. Based on your 80 minute connection you are on BA 218 and connecting to BA 518. To date through the month of June, you would have only made that connection 3 or 4 times. Unless there has been a change you still need to make conformity, which means scanning your boarding pass and being photographed 30 minutes before take-off, and that is before LHR's security check. And while BA has to take care of you, they will not be able to put you on that next flight if it is full. Given all this, I would choose that 3:10 flight to Lisbon initially, which is actually less costly, based on a random search for a Tuesday in October on Google flights. And barring a major issue you will safely make it. Note that this advice is coming from someone who generally trusts the connection times in the European airports/. The problem here is DIA operations.

https://www.heathrow.com/connecting-flights

Plug in your flight info for guidance

My advice is to look at flights from Heathrow to Portugal that fall after your scheduled flight. If, for some reason, you don’t make your connection, your airline will automatically put you on the next available flight. I would want to know the other flights that are scheduled that day. If there are a few, don’t sweat it, you’ll be put on one of them.