Share options time apportionment stages vesting

[becox]

Hello, hope you're doing great!

I'm helping a client (non-resident) to file a tax returns for share options that they exercised and which relate to their UK employment. And the part that confuses me a lot is the time apportionment basis.

I'll give an example.

Say John starts employmenet in UK on 6th April 2023 and works at ACME for 1 year.

When he starts, he's granted stock options award of 2400 options (non RCA, US employer, non approved NSOs).

The stock options award has 1 year cliff edge and a vesting period of two years. Thus, 1200 options are vested on 6th April 2024, then each month, another 100 options are vested.

John moves abroad on 6th April 2024 and works at ACME until 6th April 2025. As John vested all 2400 options, he decides to exercises.

The main question is. Based on time apportionment basis. How many options will get taxed in UK?

Option A: Given the 1200 options were vested during John's UK work and 1200 options vested during work abroad. UK should tax only 1200 options.

Option B: If I understand correctly time apportionment. Each tranche of vested options from grant to vest is time apportionment. So while abroad, the first 100 options are taxed by UK as 12/13, the next are taxed at 12/14, 12/15, 12/16.... and the last at 12/24. So UK gets to tax ~800 options that were vested abroad.

As you see it has huge implications tax wise, and the only guidance I have found so far relate to vesting in one go, not multiple stages as in this case.

Much appreciated if anyone encountered this in their practice.