Welcome to FT and the BA Board Seat11A !

As others have said it is probably not worth the hassle of going landside for the Arrivals lounge unless you have loads of time and the desire to try *every* lounge in T5.

The Concorde Room should meet all your needs however absolutely nothing wrong with trying out the First class lounge and (South) Galleries if you have an interest and want to check them out. It's actually quite a good way of killing time (and you won't be alone - we did just that on our first ex-LHR flight in First and saw a few other folk doing the same!!)