I'm thinking of moving to work in the isle of man. I

The rules around tax on the main residence are changing with effect from 6 April 2020 so I may as well go straight to those. Read about the existing rules here and the new rules here.

If you move to The Isle of Man and eventually sell the London home (after 5 April 2020), you will get exemption from Capital Gains Tax in the UK for that proportion of the gain covered by your occupation of the property and for that proportion of the gain covered by the last nine months of ownership. The balance of the gain will be subject to CGT at 18% or 28% or a combination of the two rates depending on the sum of your UK income and the net capital gain in the tax year you sell it. The first £12,000 of gains an individual makes in a tax year are tax free. If the property is jointly owned, then the gain will be divided accordingly. Mortgages are not taken into account when calculating the gain or loss on the sale of a property. Taking a mortgage on one property to buy another does not result in an immediate tax situation as you are merely taking part of the equity out early but the cots of the new property will not be deducted from the gain when the first property is sold.

You will be taxable on any surplus of rental income over deductible expenses but will be entitled to the UK personal allowance to reduce that. Take a look here for information on letting a property and the UK tax implications. You will have to register as a non-resident landlord which you can read about here.

If we have a change of government, expect a significant change in the Capital Gains Tax regime, ie higher tax rates.

I hope this helps but let me know if you have any further questions.