Charging UK users for EU roaming is most clearly a scam when you consider that Vodafone has mobile networks in almost every European country.
So you pay roaming charges for accessing Vodafone NL via Vodafone UK, but all those charges end up going to Vodafone Group anyway??? Vodafone charges you the privilege of accessing Vodafone which is then passed onto Vodafone.
EU roaming is basically a printing press for money for the three operators (Vodafone, EE, 3 UK) that do make you cough up extortionate amounts of money for EU roaming.