Is it possible to install pocket doors in the cavity of an external wall, it will be an internal door as it's between a room and a conservatory. The cavity does have foam in it though.
I don't think it's likely to meet building code requirements in the US, if you care about that. In particular, I would be concerned about both its fire resistance when closed, and the ease (or lack thereof) of opening it during an emergency situation.
If you're concerned about clearance and hence don't want a door that will swing into either adjacent room, would it be possible to use a sliding glass door instead? Those are familiar to inspectors/installers and you ought to be able to find one that's well insulated and which will fit in the same size opening as your proposed pocket door, taking into account the dead space where the pocket door sits when open.
I've never seen a pocket door in an exterior wall, but there are the "barn door" style sliding doors that might be able to be used, if you don't mind the fact that you wall they slide against can't have things hung on it, and there's exposed hardware. I have no idea how well they'd seal (and that may not be an issue, as yours sounds to be a former external wall), but with the way they're hung, you'd have the opportunity to use a much thicker and heavier door.
You could also go with @gbroiles' comment, and then frame in the stationary side of the sliding glass door to make it look more like a pocket door. (and possibly paint the glass door, or put something onto it, so it looked less modern, if it's an architectural issue)