We live in London (zone 4) and are lucky enough that DP grew up here and his parents live just down the road. As a result they are involved in our DCs upbringing, and are always on hand to babysit (providing they're not busy) if we need them to. MIL is now retired, but FIL still working full time, due to cut down/stop over the next couple of years. My Mum, on the otherhand, lives 100 miles away, so visits often, but not directly involved.
I'm a SAHM, but even when I go back to work, I still don't think i'd expect GPs to take the full load of childcare, unless they desperately wanted to- I just don't think it's fair to expect them to take on, let's face it, a knackering role at a time in their lives when they finally have more freedom. They adore the DCs, but since they have other GC as well (DPs bros have 1 child each), even if they only did childcare for 1 or 2 days a week for us, then i'm sure they'd feel that they would have to offer the same to the others, and therefore it really does become a full time job. When the DCs are older, they may well help with after school pick up and tea once or twice a week, we'll have to see how things pan out. I adore my In-laws though, and I think they really do enhance our DCs lives because they see them so often.
I really feel for other parents who i've made friends with via playgroups/NCT, who live in London, far, far away from their parents, and have absolutely no option of having their parents help, even if they wanted to. In fact, not many people I know here have any family close by to call on, not even for evening babysitting, let alone as a permanent daytime role.
MrTumble - Did the study say why children who were looked after by GPs were behind educationally? What were the other markers? Really interested, I would never think this would be the case, not sure why!