As soon as you leave the first controlled airspace, request change to the next one and be prepared to free-call or orbit if it's busy. Consider asking if they can handover you to the next frequency, although they don't normally accept due to workload. Think also about a Plan B if you don't get clearance. On the PP Weekend, I did the Western side of East Midlands and then the Birmingham Overhead. I've also done the Stansted overhead and then the Southend Overhead. It's terrific fun.