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As far as UK tax authorities are concerned - yes, you have to declare overseas income on your tax returns:
Reporting your overseas income
You must report your overseas income on the foreign pages of your tax return.
Effect of residence, ordinary residence and domicile on your tax liability
It's possible that you may not have to pay tax on your overseas income. This will depend on whether you are classed as a 'resident' in the UK for tax purposes in a tax year. The amount of income on which you pay tax may also be affected by your 'ordinary residence' and 'domicile' position.
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http://www.hmrc.gov.uk/taxon/foreign.htmSo, those with homes in the UK and spending a large part of the year there are normally UK tax residents and if so, do need to declare income from o/s rentals. There is no UAE tax to offset, but some expenses associated with the rental may be deducted.
If it is not declared, they are evading tax and are breaking the law (tut tut) - when found out they will be liable for back taxes and fines.
(PS - I got my Kindle direct from Amazon via post via Amazon.com, as Amazon.co.uk don't fulfil international orders for Kindles, and they deliver pretty much world wide, but because I use a UK credit card I have a UK Amazon account and don't have any issues with purchases as a result. I'm told, however, that torrents of drm-free books are widely available and they don't require credit cards..
)
Cheers,
Shafique