Channel Islands 2
The Migration Atlas (2002) lists the number of recoveries from these countries as 7, 1, 1, and 0 respectively.
I don't have any more information about the Belgian ones (too many for the online ringing report to list individually), except that two of them were before 1979. Apart from one in Bucks, all six from the other countries were recovered at Portland or on the East coast of England.
There have been no previous recoveries of Firecrests either into or from Wales except for two into and one out of Gwynedd; UK movements in all three cases.