TP has no flights to LHR until May 07.
BA flies daily LIS-LHR
FR flies twice daily LIS-STN.
If they can get themselves to LIS, they can get to LON.
It looks like there is a flight FNC-LIS tonight, and then there's nothing until May 05. Price-wise, the next reasonable flight to LIS is May 09.
This is what LIS looked like Sunday.