Both my daughter and wife have worked in UK pubs, and would get tips in this way. It is not common (at about £3.50 a drink), my wife might have got 1 or 2 a week, my daughter gets more but works lots in busier student pubs and is rather pretty.
In general, there is a book behind the bar for recording drinks purchased for staff, and there is a rule that the purchase is below a certain value (so no buying expensive bottled ciders, sadly, or doubles).
These drinks can be used when in the bar off shift, or taken home as bottled drinks (and thus, we end up with a whole lot of Corona breeding in the cellar when our daughter is home for the holidays). They can’t cash them out, as this upsets the audit.
Si