Rap., UK does not have any ceremonial units.
David, Russia proved itself a threat to Crimea and Ukraine, and is seen as a threat by the Baltic states which are part of Nato.
That threat is met by having Nato forces including British army on their borders.

Germany and other countries in Europe do spend less than us, relying on US to defend them.
That has been the situation since before the eighties when even you admit to Russia being a threat. Trump says he is unwilling to let that continue.