I'm moving out right now to be near grandparents and get more space. If you're commutable to London then you can come in with kids for the cultural benefits still, but without living full time with the pollution and overcrowding. That's my logic anyway.
Having visited grandparents over easter holiday I am confident that we'll enjoy a quieter lifestyle a lot.
Do you get on well with in laws? Will it be a bonus for you to be nearer to them? I think it always is a bonus for kids.
This website may be of use. www.commutefrom.com You put in where you need to be for work and the time you are prepared to travel and it churns out options. Cambridge is an awesome city if you can get over pricing. Great train to London and 25 min to Bury St Edmunds