Spotted at Tyseley works at the weekend, what appear to be insulating fishplate just like those produced in nylon by Peco.
Curious, I asked PW guru (well he knows more than me) Simon Paley what they were:
Yep, they are IRJ's, Insulated Rail Joints, those are new, but using the existing bolts, they are normal fishplates, but with a Polymer Coating, there's also a plastic plate between the two rails, much like the PECO insulated plates
On the ones in your pictures, they are on S&C so they are used in the same way we use them on a layout, there is electricity running through the rails and the IRJ creates a break between sections, although the 'Frog' is not switched.
So they really are insulated fishplates! I'm going to look for an uncoupling ramp now...