Are all these monuments cenotaphs? Or are there other reasons?
Some were Victorian vanity projects. The Stoodley Pike monument was funded by public subscription to celebrate the victory over Napoleon. Rivi was a hunting lodge. The Ramsbottom Tower was again funded by public subs to commemorate the statesman and famous son of the town, Robert Peel. Darwen Tower was built to honour Victoria’s Diamond Jubilee, and also (apparently) the victory of common people over the moor owner regarding the right to use paths over his land (which makes me feel less guilty about riding non-bridleways there this morning…).
The Aiggin Stone (halfway down this page) is interesting, a medieval boundary marker between Lancashire and Yorkshire.