Thank you for your question
My name is Clare
I shall do my best to help you but I need some further information first.
Do you still use the name given on your Birth Certificate - and where were you born?
What a nightmare for your
Can I confirm your are British - but were you British from Birth?
Do not worry - this can be resolved
have you checked to see if your birth is registered in Bangladesh?
I think that will indeed work.
What you are currently being asked for is evidence of your name as if this was your first application - and in fact as you so rightly point out you are in fact asking them to put right a mistake that THEY have made
Email them again - do not tell them that you do not have the documents they ask for - just point out that what you are asking them to do is put right a mistake that they have made and they have your original passport as proof.
Once that is sorted however it would make sense to try and trace your birth certificate!
Please ask if you need further details
You are welcome and I hope all goes well