I'm sure that there's some small print in your contract to say that they can from time to time make changes to the contract etc. I had an email advising me that the T&C of my contract were changing from an old to a new set. In the new T&C it said they could for me at least.
You need to prove the changes are detrimental to yourself in order to leave without fees.
If you have never used over 25gb when roaming you would be struggling to justify it.
Track usage, upgrade and get support with the My O2 appDownload My O2