You can't argue it's
not linked to EU membership if
all members are forced to be part of it. Yes you can join it without being part of the EU, but if you are part of the EU then you have to join it. In other words, as it currently stands, the UK has to accept decisions made by the ECHR (although I do notice that they can ignore them, as in the case of prisoner voting rights:
https://en.wikipedia.org/wiki/Hirst_v_United_Kingdom_(No_2) - and actually this clearly another example of judicial over-reach where the British parliament has no appetite for this sort of nonsense yet are being hounded by a foreign court).
And wtf is the 'simple market' ? Do you mean the single market?
And yes there are criticisms of it becoming an activist court:
https://en.wikipedia.org/wiki/European_Court_of_Human_Rights