Your BCN-LHR flight will arrive into T3, and your LHR-IAH flight will depart T5.
Assuming your luggage has been through-checked at BCN to IAH, you won't need to clear immigration, collect your baggage or go through customs. This means you can stay airside by following the purple Flight Connections signs.
On arrival into T3, follow the purple signs to Flight Connections. You will take a bus to T5. On arrival in T5, again follow the purple Flight Connections signs. In T5, you will go through security (there is a Fast Track lane on the far right - white and black sign). If the queue is all the way along the arrivals corridor in T5, don't join it - go past the queue and show your First boarding pass to a member of staff who will show you where the Fast Track line starts.
Once through security in T5 (you will be at the North search point), turn right and walk all the way to the end - you will find yourself in front of the Concorde Room entrance. Enjoy!
The T5 part of the above applies for your outbound IAH-LHR-CDG connection within T5. Don't forget you can use the Concorde Room before your CDG flight (assuming same day connection) on production of your First boarding pass.