Although I agree that Whopity's legal interpretation is correct, I wonder whether the actual reason for requiring the LAPL returned is more mundane:
Application for a PPL based on being the holder of an LAPL and having completed the required 'upgrade' requirements.
The CAA need to see the LAPL (or at least a certified copy of it) to ensure the applicant does indeed hold one - they cannot rely on their own records !!!.
Of course, if that is the case, then SRG1105A Guidance Notes need to be amended to state that all existing Licences held that are being used as the basis for an exemption to a full 45hr PPL course should be sent in (not just 3rd Country ICAO Licences)