LARS is best, ask for traffic or deconfliction, if they're busy you may only get a basic service. Failing that I'd go for a basic service from London Information. At least they'll allocate you a squawk which will tell any terminal controllers who you're talking to if they see you getting too close to their airpsace.
If you're transiting over an aerodrome call them even if you're planning to go above the ATZ - you may find out something useful about arriving or departing traffic. When clear go back to your previous unit.