UKBA’s guidance notes do not specifically prohibit travel like it normally does when you are not allowed to travel whilst an application is pending.
It does state that you must make appropriate arrangements to ensure that you can respond to their queries or requests for documents within the period they allow.
From this, one can derive that travel is allowed, but it is advisable to remain contactable at all times and ensure you are able to return without any risk of missing their deadline as this may mean that your application is rejected and will have to remake the application and pay an additional fee.
You will of course need to have you passport to travel. In order to avoid having to send your passport to the Home Office, you will have to make use of the Nationality Checking Service at your local Registry Office as they will check your documents and certify your passport copy, give back your documents including passport and send your documents away for a fee.
For further information please contact our immigration solicitors on 0808 168 5550 or email firstname.lastname@example.org.