Hi,
AI aircraft fly using GMT, not local time. So since GMT is the time zone in England those will always be correct (in the winter without DST). But other time zones can vary between FS and real life, and if your flight plan is a summer plan (during DST) and you are flying in the winter, that can make a difference too. So the actual flights can be up to 2 hours off in certain locations.
To more reliably find your AI aircraft, set AIFP to use GMT time, get the relevant time from that, then go to FS and set the GMT time (not the local time) to the same thing. Then the AI plane will be more likely to appear as you expect.
Hope this helps,