BG must have used the white HEP fittings, the old JG were obviously the originals. Get a plumber in to open up the floor and get rid of a lot of that. Or do it yourself if you're handy enough.
edited for typo
I have taken a bathroom suite out before prior to a professional installing new bathroom.
To remove this excess pipework would I just need to isolate the main water supply then disconnect the pipe work? Or would the whole system need draining?