It depends on where you are going in BD C - if it's a domestic route (UK or Ireland) then it is the grim and crowded BD domestic business lounge for you. There are no showers there.
As a *G you will have access to the T1 *G lounge which might have showers - perhaps others could advise.
If you are going international with BD then use the BD No 1 Lounge and use the showers there.