but simply because it is mandatory.
Yes it is (mandatory), but then you are confusing a pre-flight FP with an airborne FP. When you request a class D transit the request constitutes the FP. Strictly a pre-flight FP is not even required for IFR in class A, but in reality the UK never accept a pop up request, unlike, for example in the States.
The last thing we want is pre-flight FPs to become mandatory. The recent actions of some scottish airports have been described by some as a slippery slope.
A FP does have the advantage that your failure to pitch up should intitiate s&r. However, I have come back form Europe on more than a few occasions (and arrived there) to be told they dont have my FP. Bimbling around the UK it is often the case that you are never out of radio contact with someone (if you elect to take advantages of the services on offer).
All other issues aside, yes you must of course file a preflight FP for any flight abroad. (well excluding Wales, Scotland etc.
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