There is the mechanism. However, the fragmented UK ATS service provision makes it difficult and it seems that most controllers ignore the requirement to pass pilot reports on.
For example report icing at level 50 to Brize and in my experience there is little or no chance that they will pass it on to anyone.
However, remember that pilots operating in that area could be talking to some 10 or more separate ATS units.
In the upper air, ride reports ie reports of turbulence or chop are passed on by ATC to following aircraft.
Many SIGMET reports are created as a result of pilot reports. Unfortunately in the UK very few if any pilot reports when operating in class G are passed on to the met office.
Just cause the ATS system does not do it's part there is no reason for pilots not to try their best to be helpful to other aviators.
I would prefer for an accident report to highlight ATS system failures than any airmanship failure on my part.
Regards,
DFC