I think it will take about 4.75 hours to get there including breaks!
When we went there in Feb I recall we left at about 8:00 from Hyde Park Corner. We made quite slow progress out of London because we had a few punctures (because Luke was with us
). We got to Cambridge at about 1pm, so 5 hours elapsed time.
I'd expect we would be a little bit faster this time as the weather is not likely to be as bad and I think we are going on a faster route.
Here's Abs' GPS trace from the day. Best part of 9 hours moving time for the return trip!