Near Falling Foss waterfall there's a big rock hollowed out to form a 'cave' called The Hermitage. Whenever I visit I await solitude and sing The Blantyre Explosion which resounds wonderfully.
(Falling Foss is between Sleights and Robin Hood's Bay on North York Moors).
I can also recommend the Chapter House at Glenluce Abbey, Dumfries and Galloway to sing in.