As the pension pot is held in the Isle of Man, we need to review the double taxation agreement with the UK and the Isle of Man.
Isle of Man: tax treaties
Article 17 covers pensions and advises that pensions and lump sums paid to a resident of the UK, shall only be taxable in the UK. You will need to submit a request for a certificate of residence.
How to apply for a certificate of residence to claim tax relief abroad
You will need to include details of the pension lump sum, including a gross amount. HMRC will send you a certificate of residence, which you should send to the Isle of Man tax authorites, to claim repayment of any tax deducted from the lump sum.