PhilD,
Just a bit of clarification re transitting Gatwick.
The restriction to fixed wing VFR transits only applies to the aerodrome traffic zone, the rest of the CTZ/CTA is fair game for transitting VFR (subject to the usual caveats atco's like to trot out about workload, other traffic etc).
As far as transitting the Class D IFR, well yes if you want you can request an IFR transit. Unless you really have to though I would always recommend requesting a VFR transit if conditions permit because there is much more flexibilty in terms of what separation (or lack of) needs to be applied by the atco.
For a transit, either VFR or IFR, all that is required by way of a flight plan is a call on the r/t a suitable distance from the Class D boundary.
WF.