I believe from when I last looked into that 8.5 is the best I can aim for without having more work done than I have the time and probably money for.
We have an amazing Bristan one in the en suite that heats up brilliantly and has good pressure but they have discontinued it. All those I've had in the main bathroom are utter shite and give dribbles of hot / cold water.
Any recommendations gratefully received so DC will stop using my bathroom!