What to do to fill the time?

We're having a slightly rushed wedding due to family illness and have managed to book our registry office a reception venue. Unfortunately, we were only able to hire the registra and venue from 11:45 till 12.35. The reception venue hire starts at 3pm. My dilemma is what do we do to fill the time? We have a few guests coming from outside London and they may not have time to have breakfast so they probably won't be eating till roughly 4pm which is a long time. I'm also conscious of drinking too much as we're also having unlimited drinks before the meal. The ceremony venue is near a park so photography will take roughly an hour but I'm still at a loss as to filling the time. I don't want guests to wonder whats happening or be starving or get drunk too early. Suggestions?
We did consider hiring a routemaster but they charge £600 and were not really near any sightseeing locations.