O2 is having to bat away criticisms from some media outlets that EU roaming charges are returning post-Brexit. 

The network is indeed introducing a charge, but only after a 25GB allowance has been used up. So it's not a return to the expensive roaming charges of a few years ago. Within the EU, operators are still permitted to do what O2 has done and implement a 'fair use' limit. 

O2's subsequent charge if you go above the 25GB limit is £3.50 per GB.

That cost is, however, more than the EU permits within its rules: "If a person reaches the limit, they can continue to use data while roaming for a very small fee: reduced to 3€/GB (+ VAT) in 2021 and to be further reduced to 2.5€/GB + VAT in 2022."

So if you're a really heavy data user and go above the 25GB limit, you will indeed be paying more post Brexit than you would have if the UK had remained in the EU. 

O2 told Pocket-lint in a statement that "Less than 1% of our Pay Monthly customers reach anywhere near 25GB during occasional travel to Europe.

"If a customer’s UK monthly data allowance is over 25GB, from August 2 they will have a Roaming Limit of 25GB in our Europe Zone. This means they can use up to 25GB of their allowance at no extra cost – we’ll text them if they get close to the limit, and again if they reach it.

"A customer can still use data if they reach our Roaming Limit, but will be charged £3.50/GB."