I flew TAP LHR<>LIS a few months ago, though I normally fly BA.

Their Y class product is about the same IMO. TAP will have annoying piano-covers of cheesy 80s pop music playing while boarding though!

The BA First Class lounge at LHR T3 is excellent and quite new, the *A lounge in T1 is also good.

However, the *A lounge at LIS will almost certainly be better than the third-party BA offering, so this may swing it if it is important to you.

If I were you I'd go TAP due to the frequency of flights.