It's worth noting that there is a chance that you will not have to go to T5A to clear security. Long-haul flights are more likely to arrive at T5B or T5C, and there is a chance that your onward flight will also depart from one of these satellites. There are posts in that thread about using the security channels at T5B or T5C if that proves to be the case for you. As soon as you disembark your inbound aircraft, there will be screens that show you which gate has been allocated to your onward flight (if that has been done by then), so you will know whether or not you can remain in the satellite without having to go to T5A.