Today's excellent British saying is the excellent 'flogging a dead horse'.
To flog means to beat or hit repeatedly, and the expression 'flogging a dead horse' comes from the old practice of beating a horse to make it move faster or work harder. As a dead horse can neither move nor work and there is obviously no purpose to beating or flogging it, the expression is used to point out that a particular activity or action is completely pointless, hopeless and/or impossible - in any case, a complete waste of time and effort.
'He keeps trying to pass the test, but I think he's flogging a dead horse'.
'Nobody's interested in going on a trip to the theatre. She's flogging a dead horse trying to organise it'.
Have fun describing things that are a waste of time! But please don't flog any actual horses...