It's not clear from your question whether you have been disclosing your UK rental income to HMRC in the UK as it is taxable in the UK notwithstanding the fact that you live on the Isle of Man.
As far as selling your UK property is concerned, so long as you complete five full tax years as a non-UK resident for tax purposes you can sell it and you won't have to pay Capital Gains Tax in the UK. The five year period started from the first 5 April following your departure from the UK. If you sell the UK property before you complete the five full tax years as a non-UK resident, you will only have to pay CGT (assuming there is a taxable gain) should you return to live in the UK before the five year qualifying period is completed.
The gain from selling the property will be calculated by taking the disposal proceeds and deducting from them the purchase price, the costs of purchase, the cost of any improvements (costs claimed with no receipts or invoices in support may be disallowed) and the costs of selling.
Assuming the property was your main home, you will be entitled to exemption from UK CGT for the gain for the period that it was your main home and for the gain for the last 36 months of ownership. You will also be entitled to some letting relief. I'd be surprised if there was a taxable gain after those reliefs but that would depend on how long you owned and lived in the property and what you sell it for. This will all be academic if you do not return to live in the UK before the five tax years of non-UK residence are completed. Take a look at the HMRC helpsheet HS283
for more information on the main residence and CGT.
I hope this helps but let me know if you have nay further questions.