Seats 3500 and it was half full.
To be fair, their first album had hit in the US by then and they had been on their first tour for a few months. They were the supporting act in the US, but headlined in Europe.
I was there with my family dropping my sister off at the BYU London Centre and saw a poster nailed to a lamp post about Van Halen playing a short distance away, we got tickets same day.