London landlords are the most informed sellers of Dubai to themselves. The arithmetic they already know at home, stamp duty on a second property, mortgage interest relief restricted to a basic rate credit, income tax on rent and capital gains on exit, is the reason the enquiry happens. Dubai charges a one-off four percent Dubai Land Department transfer fee, no annual property tax and no tax on rental income in the UAE, so the net yield comparison against a London flat is not close.
Rules that apply to every buyer in United Kingdom are covered in the full United Kingdom buyer guide. This page covers what is different in London.
In Dubai there is no annual property tax and no UAE tax on rental income, and the main entry cost is the one-off four percent Dubai Land Department transfer fee. In London you face stamp duty at second-property rates on purchase, income tax on rent with mortgage interest relief restricted to a basic rate credit, and capital gains tax on exit. UK residents still report the Dubai income and gain in the UK, so the advantage is the absence of a second layer of tax, not the absence of all tax.
If you are UK tax resident, yes. You are taxed on worldwide income and gains and must report the UAE rental income on your self assessment return, and any gain on disposal. The UAE does not tax it, so there is generally no foreign tax credit to claim. Confirm your position with your accountant.