The requirement is to undergo a training flight with a FI(A), which is JAA terminology, the US equivalent being CFI or CFII. It is pretty clear to me at least that the intent is that this shall mean a JAA FI(A). Ultimately, of course, it is a matter of interpretation and, since JAR-FCL has no legal force in the UK beyond that granted under the ANO, it is only the CAA's interpretation that matters and there is no doubt, given the LASORS quote, what that interpretation is.
The matter of ICAO instructors training for JAA licences and ratings is a complete red herring as they have to be authorised in accordance with Appendix 1 to JAR-FCL 1.300, which restricts the privileges to instruction "to those parts of the ATP integrated course where the instructor can demonstrate the experience relevant to the intended training". Allowing the privileges to be extended to instruction on other courses was a decision taken unilaterally by the UK in the face of stiff opposition from some other JAA member states and is not compliant with JAR-FCL.
Part-FCL will allow a little more flexibility in this area. Rather than an authorisation, it allows for a FI certificate to be issued to the holder of an ICAO licence, thus conferring all of the privileges of an EU FI. The privileges may be exercised only in an ATO located outside the EU but, within that environment at least, the instructor would be able to conduct the 1hr flight for revalidation of an SEP rating. However, it will still not be acceptable for an instructor who does not hold an FI certificate issued in accordance with Part-FCL to provide that instruction.