If you can evidence that you hold indefinite leave on arrival back to the United Kingdom then it is unlikely that you will encounter any serious problems at immigration control as once it is clear that you hold indefinite leave you will be allowed entry as of right. You might be asked questions by an immigration control in order to establish your status and histroy if it is not immediately clear, hence why you should carry your previous passport to evidence this if necessary. Ideally you should apply to have your new passport endorsed with your existing grant of leave. This can be done at your local Home Office Enquiry Unit depending on where you live. You can apply in person and pay the premium fee of £578.00 and the endorsement can then be done on the same day.
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