The RPS will always be on the safe side and should be used for terrain clearance unless there's a more accurate local aerodrome QNH available. But it provides little in terms of separation from other traffic below the transition altitude; above that then SPS will help, but it isn't mandatory except uder IFR.
And to whom would you send a PIREP in the UK? Do you really think an Air Trafficker would have the time to pass it on? Unlike the US with its PMSV system, there's nothing similar in the UK.