Earlier this year, NATS launched a portal for GA pilots wishing to transit Class D controlled airspace in 
the London area. It allows GA pilots to pre-notify their intentions via a simple, online tool.
The tool is currently a trial and use is not mandatory; indeed, it is perfectly OK to cross class D airspace by filing an airborne flight plan, 10 minutes prior to entering CAS, which you effectively do by calling them up and asking for a zone transit. By giving advance warning (1 hour before crossing), NATS say that it allows controllers to plan for your transit.
Some tips for planning a class D transit:
- Have a route in mind, taking into account the runway in use, which you can work out from the wind direction or by phoning an ATIS telephone line (ATC often prefer you to transit via the landing threshold)
- Plan your route and altitude so as to avoid wake turbulence. You want to be above the approach path of inbound commercial aircraft (a threshold transit does this)
- Familiarize with VRP’s shown on the chart, and other features which ATC may reference (the Diamond hangar at Stansted, for example)
- Be professional in your RT calls: be concise and complete (CARPACER), no waffle, read back accurately
- Have a back-up plan ready, in case the transit cannot be approved
- Fly the cleared routing accurately (heading and altitude) – this reassures ATC that you know what you are doing
- Be ready to fly faster than normal if ATC request it (though in our GA aircraft, you won’t have much speed upside!).
I have used the tool several times. The functionality is quite basic – it doesn’t save data, nor does it allow you to specify your transit route (you simply state which class D CTR you wish to cross). The tool doesn’t guarantee that transit will be approved, but so far it has worked for me even at very busy times.