Just wanted to share my unfortunate customer experience in the hope that the issue may be addressed.
I ran out of data so I went to MyO2 App and changed tariff for a higher data allowance. I immediately realized after that the change will only take place next month so I was still out of data. Tried to buy a data Bolt On but the app would not allow stating that I have just changed tariff. Went back to cancel the tariff change but the same message appeared. Rang O2 customer service and got a link to buying the data Bolt On with the same error message. Could not get through to speaking to an assistant since their operations are reduced due to Covid.
In conclusion I am still out of data and have no way to upgrade. Thanks O2, looks you clearly don't want my money