After understanding your residence/domicile status, the remaining points to consider are:
1. Do you have assets with unrealised gains that could be sold or rebased before you relocate or assets that would be taxed punitively after you relocate?
Both the UK and the US have a tax on capital gains and they both have rules that target certain types of overseas investment, making them unsuitable once you become resident there.
2. Do you own real estate in your country of origin and what do you intend to do with it when you move?
If you wish to sell property in your home country, it may be advisable to do so before moving. If you plan to keep the property and rent it out, you should consider how the rental income would be taxed in the UK or the US after you become resident there.
3. Do you intend to work after moving and where will you be performing the duties of your employment?
Consider how your income will be taxed, whether social security contributions may be payable by you and your employer, and what exemptions might be available with the right planning. If you own a business in your home country, the profits may become taxable in your new country of residence, regardless of whether you take a salary or dividends. With careful planning before you move, it may be possible to save some tax.
4. Both the US and the UK have an estate tax.
If your assets have a total market value on the date of your death exceeding any exemptions that may be available, your estate will be taxable. Currently, both jurisdictions levy a tax at 40% of the estate, which can significantly deplete the value of your assets as you pass them onto the next generation. For many, this will be the most important consideration for long-term tax planning.
5. Are you the grantor/settlor or beneficiary of a trust?
If so, you need to understand how trust income and gains are treated after you become resident and what tax-saving opportunities there may be with the right planning.
6. Finally, you’ll probably need to file tax returns once you have relocated.
This is likely to be complicated and it’s important to ensure you’re fully compliant to avoid significant penalty exposure.