All UK STAR charts have a box somewhere on each page that states:
Pilots should plan for possible descent clearance as detailed in the table below (or similar text, this example from the OCK STAR into London Heathrow)
ACTUAL DESCENT CLEARANCE WILL BE AS DIRECTED BY ATC (this part looks to be the same on the charts I have viewed for Heathrow, Gatwick, Manchester and Glasgow and is always written in capital letters)
Follow this link here
to the Aerodrome UK Air Pilot if you want to view the STARs yourself.
To answer your question, only descend when you are directed to by ATC when following a STAR chart.