Hello - I was waiting to see if anyone else answered your post. I have no experience of Parkside, but ...
I think what you're asking for is a comparison of London schools v. Parkside - and I do have experience of London schools.
As Lilymaid says, you'll find it hard to better Parkside - but i guess you already know that.
Thing is, you can't base such a huge decision on schools alone, not really.
But - moving to London ... . Where will you live? From my limited experience, it is just quite tricky. And it might be a real nightmare. Although you can't make a life-decision based on schools, you'll almost certainly want to put schools into the equation if you decide to move. So then it becomes an issue of moving to London - into an area with a decent school. And finding a decent primary and secondary in an area you want to live in (ie. one that compares favourably with what you have in Cambridge,), well, that is a real tough one.
My sympathy to you. Fwiw, we looked at moving in the opposite direction - from London to Cambridge - but eventually ruled it out on the grounds that the commute back into London for work would be a nightmare. But it was very appealing ... .