Gleanings from "meeting with remarkable trees"
There are 50 gargantuan (over 30 feet in circumference) found in British churchyards. With this sort of diameter they can be presumed to be over 1000 years old, because of our damp climate their heartwood rots away leaving cavernous openings.
The Tandridge yew - Surrey. Evidence found by archaeos that a saxon vault under the west wall, a relic of the first church, was deliberately skewed perhaps to avoid the roots. Could be that it went back to "celtic" times.
As a xtian symbol, and probably a celtic symbol as well, the tree faces two ways. Firstly it encompasses life as something that goes on for ever symbolising immortality. but it also represents death with its poisonous berries. Its warlike symbolism in that its good for spears and bows I leave to others..
Also of course its evergreen, as is the holly and the ivy.
A visit by G.Cambrensis in 1172; "Yews are more frequently to be found in this country than in any others I have visited; but you will find them principally in old cemeteries and sacred places, where they are planted in ancient times by the hands of holy men" or, of course the yews had pagan origins
Theres one in irish celtic lit. the Yew of Ross, probably a tribal tree.