I was a bit taken aback by this piece of news, Southwark Cathederal is not exactly a Conservative Christian place and is known to be a bit daring. However what exactly is wrong with the hymn? It doesn't say that those feet in ancient times walked upon England's mountains green. It says "And did those feet in ancient time walk upon England’s mountains green?" It is a hymn about trying to live out ones faith within a nation, to try and propagate those values. It's the one hymn which makes me personally feel spiritual and patriotic at the same time.
That said, I appreciate it is open to interpretation and if the Dean, as well as the congregation, have a conscience about it, then fair enough