Thus we have the UK 450Kg upper limit for microlights but elsewhere in Europe, the limit could be higher. This means that if you can get your hands on a 460Kg aircraft registered in a country that allows this to be a microlight, you can indeed fly a microlight and have the hours count towards the requirements in the UK for CPL license issue.
Is this really true? If, for example, you would want to fly a microlight in Germany (where the MTOW is 427.5 kg), then you would need a German microlight licence to do so, in which case you would not be exercising the privileges of your JAR-FCL licence....
In any case, and from personal experience, its worthwhile remembering that microlight aircraft and pilot certification requirements are regulated on a national level, and that they may (and very likely will) therefore differ from country to country! The same applies to crossing borders in microlights, where e.g. the Danish requirements are extremely strict and awkward.