Often1

Yes. But, you will also need to read the contract you agreed to with the OTA. Those third-party vendors often impose conditions above and beyond the fare rules. It goes without saying that if the BA fare rules are wholly inflexible, you are free from that secondary assignment.

Globaliser

That suggests that that is what you agreed to when you bought the ticket. Do you have screenshots of the fare rules displayed to you when you bought it?

Palty

nope I don't

Globaliser

Assuming that none of the other documentation sent to you by the OTA contains the fare rules, either, it's then going to be quite hard to find reliable information about your fare rules by any alternative route. In other words, you're likely to have to just accept what the OTA now says to you about the fare rules.

An OTA may or may not have sold you a BA published fare - you can try to work this out by taking the base fare of your ticket and comparing it to the fares on the fare ladder for your ticketing date, which you can get if you have an ExpertFlyer subscription, and thence go to the full fare rules - but if you went to an OTA because it was cheaper, then it won't have been a BA published fare and it's far from guaranteed that your fare rules are identical to the rules of BA's published fares that day.

At any rate, what we can say is that if the OTA says that you can't make changes to your ticket, that's consistent with what BA's own fare rules would have been for an I class ticket: no changes means no changes.

itsmeitisss

Are they on your confirmation email?

ricktoronto

Ex-UK means it's leaving from the UK vs. say France.