If you are coming in on an international flight, and are on a through ticket, you will stay airside and have a security check but no passport control. Just follow the signs for connections. On the basis you have no status you will have access to two lounges at T5A, Galleries North and South. There is another lounge a T5B but it is likely your flight will leave fromT5A. You will have plenty of time to shower. Someone more knowledgeable may chip in with the practicalities as I have never needed to use them.