Every area has it's faults. You either end up hitting against a big body of water, or a military base. Or in some cases, both.
Do London, and you will eventually find things like Heathrow and Gatwick are a problem. Solve these and you're gonna hit the military bases further out.
Its a similar issue for me here. I can get a 23x23 pretty easy,but I then get stuck between the Ijselmeer and the North sea. At which point I have to give up on what I've done north of Amsterdam Zuid, and start working down, but even then as I go east I'm going to hit various military camps. There comes a point where you basically hit the limit of max square, and have to aim for max cluster instead. Unfortunately.
J