So let's see, roaming charges are generally free just now due to reciprocal agreements between countries, so there isn't as large fees to pass on to consumers as before ...
No, let's say the EU decides that as the UK is no longer part of the EU, it will increase the roaming charges to UK mobile users by €€€€, hands up who thinks the mobile operators will just take that on the chin and soak it all up and lose vital revenue for their customer ..
Even if the UK matches those rates, good luck to any operator who thinks it'll even out or they'll make a profit