The US tax code places an indefensible burden on expats. Also, some States (looking at you California) can require you to pay income taxes even if you live overseas for the entire year.
Other factors to think about:
- not all countries have tax treaties, the default is you owe in both places
- tax deferment schemes complicate things (e.g. 401(k)), the countries won't look at this income the same way
- some tax treaties require specific actions, missing these may leave you owing
- not all income types will be treated the same way in both jurisdiction (e.g. cap gains, rental properties,etc.)
- etc.
Personally I think there is a high dollar value in just not having to worry about these kinds of things. If I were entering a complex working situation or moving to another country, I would probably take steps to make things as simple as possible such as ending work in December and starting something new in January, not realizing investment income, putting assets in corporations or trusts to defer any income, etc.
I do fear state tax filings are about to get more complicated with some states apparently looking to collect money for even one day business trips. I wish Congress would get off the blocks and pass the minimum 30 day exclusion that's been brought up a couple times.
The US tax year runs from Jan - Dec, with a quarterly estimated tax payment schedule for self-employed people. In the UK, it runs from Apr - Mar, with bi-annual tax bills. So there's a point at the end of the calendar year where you owe US taxes (that you should be able to fully wipe out with tax paid to the UK), but the UK hasn't calculated the tax amount yet nor asked for the money yet, so you can't show the US that you already paid those taxes under the tax treaty. So you end up owing two countries taxes on the same money, at least temporarily.
tl;dr - everything sucks, hire an accountant
It's also entirely possible that the answer you get is "well, this is how we interpret the updated tax treaty, but nobody has tried it in court so we aren't sure."
Usually this stuff is in the details rather than the basic question, but it can be fiddly.
A lot of it comes down to the accountant saying "our firm recommends X as more defensible based on recent changes in legislation, but if you want to do Y, here is why that might also be possible."