With sacred wells it's a tough call. I think that each one has to be taken on its own merits. An old raggy tree is usually a fair sign of age, but there are some new ones around.

In Ireland there isthe luxory of checking who the well is dedicated to. If it's St Brigit then your looking at a pagan well. If it's the local saint then it's not so clear, but the day of the patron can be an indicator - is it near an equinox, solstice or other pagan festival. Bullaun stones by the well are another sign of pagan origins.

In the UK there is often only local knowledge and folklore to go by.

Wells at fords or bridges are often pre-christian. Offerings would be made to the water before crossing it.

Yep. Wells are tough one, but there is a high proportion of them that are of pre-christian origins, so if they are visitable then they should be included. Although one visitor may not see something about the site that screams 'AGE!' another might.

I suppose the best thing is for people to say why they think a well or spring is worthy of inclusion with details of folklore or nearby remains etc.

You're definitely right about one thing. A website dedicated solely to the holy wells of the British Isles could be massive.