Had to unscrew mine to get hair out of it last month(wasn't a nice operation).
Anyway that screws to the pipe underneath which I assume means you can buy a new top bit for a couple of quid. Helps if you have access to the underside of the bath to hold the pipe up too, otherwise you have to use string and hope...
iirc that whole piece should just unscrew, no idea what it would be called to find a replacement though.
grab the prongs on the plug at twist
Only the centre bit flips off (for cleaning), to remove the surround you will need to get access to the underneath, disconnect the piping and unscrew the large plastic nut holding it to the shower tray. "Shower waste" I think it's called.
To refit, apply silicone (anti mould type) around the area and screw into place.