London has some great venues for classical music - the Royal Festival Hall being my own favourite. The music is its own reward but it is also great to meet some like-minded people before a concert, and/or after, for a drink and a chat. The format is simple - show up in a corner of the bar from 30 minutes before and immediately after the event, lubricated as you feel appropriate. Find your kindred spirits and interact.
If we ever require a recognition signal in a crowded place, then displaying the QRcode for this page (our present group photo, till we have something with people) should serve the purpose.
I see this as a way to add some conversation with like-minded people to the pleasure of attending a concert. Having decided to attend a concert, why not list it as an event here? Then if several of us are attending, we can get together, if we like. Simple :)