The aircraft you are seeing apparently behaving erratically are ones that don't send their position via ADS-B, and so FR24 uses a technique called multilateration (broadly analogous to triangulation) to calculate their locations.
Multilateration (MLat) requires extremely accurate time synchronisation between each of the receivers involved, otherwise positional calculations are prone to significant errors.
I would expect that there are many more FR24 MLat receivers in the Manchester area than in Kerry, so I guess it's possible that synchronising them all presents more of a challenge.
That's my theory, anyway.
