Post
by secret.simon » Sat Feb 07, 2015 8:29 pm
The UK does not place any restrictions on your use of your old passport. However, the country that issued the old passport may have restrictions on its use. For instance, as jaweb has mentioned, an Indian passport should be surrendered within three months of the citizenship ceremony. But that is Indian law, not British law. Similarly, the US requires you to enter the US with its passport if you are a US dual citizen, whereas the UK does not have any such limitation (i.e.: you could enter the UK with a non-UK passport that has been appropriately endorsed).
So, it is down to the country of issue, not the UK, to decide how long the old passport can be used.
You can travel while the application is being considered assuming that you have your passport (and BRP, if applicable) with you.