Non-resident taxable where?

I have a client X (UK national & UK domiciled) who is currently non-UK-resident as he has been on a full-time employment contract in the US since 2008.

This contract has now ended and X is to receive an "ex gratia" payment for having got a good price for the owner Y on selling the business that he (X) was running for him (Y) in the US.

I believe the ex gratia payment will be taxable as employment income (unless anyone can tell me why it would not be - his job was to add value to the business with the probability of a sale) but he (X) does not want the ex gratia payment to be made in the US because of exchange rates on transfer to the UK.  It will be paid into his account in the Channel Islands, whence he can apparently transfer it to the UK at a much more favourable rate (for whatever reason).

Where is this likely to be taxed?  The employment has ended, but my view is that it is probably taxable in the US as he will probably still be resident there when he receives it.  He reckons it could be taxable in the UK because that's where it will end up.

Any help on this complicated issue warmly received (and yes, I have posted another question on another issue concerning the same person) - thanks!