They are at many stations as a back up to the train's own screenwash and wipers and overnight cleaning on the depot.  Useful for when a large number of insects are around - for example Mayfly nights where I have seen trains with insects coating the front of the train several centimetres thick - a test for any wiper system!

Complications with electrification mean the long wooden brushes can't be used at places like Reading, but shorter alternatives (and a multi-page instruction on how to use them safely!) are in place at places with OHLE^▸ (Over-Head Line Equipment (electrification via catenary)).

Also the open containers with screenwash in were getting bad press as a health hazard (no, it's not drinking water Mr. Dog!) so they tend to be in more secure containers these days.

Both of those things mean that they aren't used anywhere near as much as they used to be.