Thanks for your response
You should have completed a P85 as soon as you were due to leave the UK so HMRC could establish your residency status for that year and subsequent tax years - I would rectify this as soon as possible - link here for the P85
This provides the option of completing online (if you are registered for online services) or to print off a paper copy, complete and post to HMRC
You only are issued an NT code if you have a source of income in which to use it against, which you have and will not (as the SIPP is not a income that can be taxed/paid tax free through the PAYE system) so this is not a viable option unless you make a test withdrawal initially after age 55 so that it highlights on HMRCs radar thereby allowing a PAYE record to be created by HMRC and a tax code issued (NT) But this is a long shot.
Instead you will normally have to suffer the tax deducted (code OT so the correct rates are applied) and then claim a tax refund through self assessment (with a pension part of the main SA100 return and also complete SA109 residency and remittance) which then should see HMRC reviewing for any refund due.
But usually with SIPPS you are only permitted to take a certain amount per year - so do check with the SIPP provider. And if this proves to be an issue then a suggestion that a consideration of transferring the whole pot into QROPS - but do take pension advsie and also what terms and conditions apply with accessing funds and the timeline relating to this.
Tis should be a transfer that can be made tax free - then fall out of UK jurisdiction -
BUT ensure it has the flexibility that you require and the ability to action also at 55
(I am sure you can appreciate we are tax experts and not pension or financial advisers!)
Do let me know if I can assist further - or, if you have all that you need, then it would be appreciated if you could rate me for the level of service I have provided (or click accept)