I too will be interested if you get many responses about spider mite control.
I, like most people, get occasional outbreaks, and have used Neem oil, emulsified in washing up liquid, as a spray. This does seem to sort it, but though Neem oil is imported here, it hasn't formal approval for use as a pesticide. That gave rise a few years ago to rumours that it was on the "to be banned" list in the EU. But it seems that it can't (shouldn't) be sold as a pesticide without the appropriate licenses. Other than that, I don't think there is a specific acaricide on the UK markets, though some Provado sprays suggest that they control red spider. Not the concentrate so it gets expensive if you had a large collection to spray regularly.