I have just heard from a friend that if you have dual citizenship, you will have to have "NTL" stamped on your passport (not the British one) before you can travel out of Britain. I have two passports: one Nigerian, the other British. I do not have "indefinite leave to remain" stamped on my Nigerian passport because I was given indefinite leave to remain as a refugee more than 15 years ago; at that time, passports of refugees were not stamped. What do I do now? Are there new rules about this? Do i need to stamp my Nigerian passport? This is very confusing for me.
There isn't a passport check to get out of the country.
The airline will generally check your passport to make sure you have permission to enter the destination (they have to bear the cost of returning you if you are refused entry) and the staff at security and boarding will normally check some form of Id matches your ticket - but mostly to make sure you paid for a flight.
How would they know you also had a Nigerian passport ?
If you are flying to Nigeria and present your Nigerian passport to the checkin staff they might notice that you didn't have an entry stamp to the UK, assuming you entered on your UK passport, but they shouldn't care.