Large groups? - You get the occasional cluster of 10 riders but I suppose groups of three or four is more common. It is certainly a very social ride so it's rare to see people riding on their own for very long even if they started riding solo.
Average Speed? - How fast do you want to go? There are no timing chips, rider listings or gold medal times so people just go at there own comfortable rate. An important consideration is the cafe doesn't open until 5 (or is it 6?) so if you put in a record-breaking time and arrive at 3 you're just going to be spending a few hours getting cold waiting for the cafe to open. My plan is to leave at 10, have an easy ride, and arrive in time for a Full English at 6.
Lights? - once you're out of the city you're riding on unlit country roads. A set of £400 off road lights is overkill but you want something with more poke than a flashing commuter light. Having said that unless you deliberately ride on your own you can often share the pool of light provided by your fellow riders.