Yes, I do have a Surestop but I'm absolutely very dam-ned sure I didn't pay that much for it!!!!!! I use it regularly, though not on the mains feed - I use it for an outside tap and it has worked perfectly for at least the last ten or more years. The o/s tap is fed from the plumbing to a concealed toilet cistern in a downstairs shower room by drilling through the wall to the outside and provides a tap to a part of the garden that would not otherwise have one.
It consists of a 15mm push-fit plastic valve assembly that is inserted into the water feed to the tap. (Like a speedfit fitting, it can be removed/relocated by pushing the end rings inwards and I have moved/replumbed it several times, which is another story).
From the valve a hollow plastic air tube is run to the SureStop switch, there was plenty of length in the kit, my criticism is that the switch I have appears to work upside-down. It is entirely mechanical, no electrics needed, in winter, I isolate the o/s tap by pressing the switch, and it works. The tube is buried in the wall under the tiles.
However, OP doesn't have a lot of space to play with and some thought will be required. But it can be made to work, so that OP isolates his mains feed by flicking the switch that can be located anywhere that the tubing will reach (and labelled as such).
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Paul