I may be wrong....how often do i say that?
I have always thought Peridotite was the oldest rock in the UK...you can find outcrops of it on Skye.
Talking of garnets...I have a matchbox somewhere full of the little devils collected half way up a mountain in Knoydart...just picked them out of the rock with me penknife.
I seem to remember there is a quarry half way along the Malvern ridge famous for fossils...which brings us back to amonites...
MrH