Emo, are you on the Mirena for heavy bleeding or unpredictable/uncontrolled bleeding? Or just because it appealed to you for other reasons?
It would be difficult to recommend you coming off if you went on for bleeding because it does control bleeding really well and because utrogestan often doesn't, if women are prone to having bleeding issues on HRT.
But if you went on it just because you liked the convenience or something, you could try switching to utrogestan. If you want to try the utrgoestan-only thing, get the book by Lara Briden called Hormone Repair Manual. And know that your GP may resist this with "computer says no" because it's not a way they really use it, by itself, in the UK. So you might need to at least accept a script for estrogen and then just get the progesterone...