If it is on one ticket, LH should be able to issue the boarding pass for TG at BCN.

I just did LH903 LHR-FRA to TG921 FRA-BKK a few weeks ago. LH at LHR check in was able to issue my TG boarding pass.

Not sure if Spain is part of Schengen. If not, it will most likely be similar to my experience on arrival from LHR. We parked at a gate that the bridge could not be used. We took the bus around to somewhere. When we alighted from the bus into the building, immigration was on the right and connecting flights on the left. Went up some escalators to the Skyline train platform that took us to the B area. Walked some more, went through a near empty security checkpoint and I was in the B area. Spent about an hour at the Senator Lounge near gate B42 before heading to board my TG flight. No immigration.