But funnily enough, T-Mobile NL is the only Dutch provider who does not count Switzerland (a non-EU state) as an EU member so if, for example, you drive from the Netherlands to Italy you will face roaming charges for the Swiss bit of your trip. As for the grandparent: mobile operators were indeed forced by the EU to abolish roaming charges, so you'll have to wait until Serbia and North Macedonia join the EU, or they get so far in the process that the earlier agreements already provide for the abolition of roaming charges.
As for the British operators reinstating roaming charges, this was obviously always going to happen. When they were forced to get rid of them, they were happy to take the credit, but it was always EU rules that forced them to and many operators throughout Europe complained and conjured up nightmare scenarios about how this would lead to higher prices for consumers, but as far as I can tell this hasn't happened anywhere in the EU.