Isle of Man

[BlasterBates]

There are different types of income. Basically:

Employment income: this is taxed once and only once. If you have a permie job in Paris and keep it for one month and return you'll pay French tax (might not be much and could potentially be 0), and you won't be taxed again in the UK, regardless of how little tax you paid. You will be classified as French tax resident for the month you have your permie job.

Dividends and Investments: These are usually taxed twice, according to credits, as described by ASB. You'll end up paying the highest tax rate if you're resident in two countries.

Corporate Income. Companies operating in two countries will only be taxed once on any income.

Ok a bit of a simplification but a good rule of thumb.

[expat]

If you come to the UK, your time here is now measured by number of midnights. not the older formula of number of days not including arrival and departure days.

Becoming resident in another jurisdiction does not automatically stop you being resident in the UK, for which the usual tests apply, including but not limited to number of days in the UK.

Double Tax Avoidance treaties exist to try to ensure that you do not have to pay two full sets of taxes on any one period of earning, but:
1. You do not get to choose whose tax you pay, the tax people decide that. They may disagree amongst themselves, and you will not find that convenient.
2. You are quite likely to have to pay one set of tax, then have to pay the difference to a higher-tax country, i.e. one country credits you with what you actually paid to another, as against letting you off completely because you have paid another country for that income; though it can happen that way.
3. The DTA formulae genarally try to tax you at a level consistent with your aggregate income, to stop you from earning a lot but over many different places. If you're rich, everybody wants to tax you as rich.
4. Generally speakn=ing, as has been mentioned, regardless of residence, if you earn money in a country you should expect to pay some tax to that country. As indeed you should: if you don't want to pay a country's tax, stay out of its economy.