The £50 pan and cistern from Wickes is fine. Having fitted 2 of them in my house they seem to do what's needed, with no issues.
What I did do was dispense with the fittings that came with the setup and install Fluid master fittings as shown in my initial post.
My reason for doing so was if there is an issue with the flush valve or inlet valve in the future. I can go to Wickes or Screwfix, buy a replacement and just plug the replacement straight in without even turning the water off.
The flush valve that comes with the £50 toilet set is more than likely some cheaper obscure valve. that's why they cant identify it at Plumbcentre.
Don't mess about, turn the water off, drain the cistern down, disconnect the water supply, undo the two butterfly screws and lift the cistern off.
Fit the new valve making sure it doesn't rattle around in the hole ... if it does, fit the reducing collar. There are only two sizes 1 1/2" and 2". It is included with the valve... and don't over tighten the large nut that holds the valve in place.
Good Luck