I don't know what a BT fare is, but you need to look at the detailed fare rules on your e-ticket. Fare conditions depend on the fare bucket you're in, not at all on your past experience. If you're confused about these you can post them here for clarification, but usually out of the UK there is a 75 GBP change fee and fare difference for that date is payable.

Sometimes fare buckets open up at a later date, so one option is to tell EK you won't be traveling on the original date and reticket when fares go down.