Ok so you are married in both Indonesia and UK and from what you mentioned these 2 mariages are not connected, they are basically 2 mariages.
There is already a divorce in place for your mariage in Indonesia but this mariage is not registered in the UK:
- so first, I don't see how you could divorce again if there is already a divorce in place. But it's Indonesia so anything is possible.
- but second, if that marriage was not registered in the UK, I don't see how the British embassy can accept the Indonesian divorce to enforce the divorce of your UK mariage.
To me, if the Indonesian mariage was never registered in the UK but you actually had another mariage in the UK, then you need to divorce from your UK mariage. It's not ideal but that seems like the most logical way. If you go through all the troubles of getting a divorce letter of your Indonesian mariage only to be told that it's not recognized because never registered, you might be wasting time to end up in the same position anyway.