It is my understanding that SMP is payable no matter where you are, provided you meet the eligibility criteria to receive it from your employer in the first instance.
My concern would be if your ILR expires, you are technically no longer legally employable in the UK and therefore should you really be on the payroll system of a company in the UK while they pay out the rest of your SMP? I would have thought they would be forced to terminate your employment at that point. It may be that they pay the outstanding SMP at that time - I suspect you would have accrued annual leave they might also need to pay at the same time. I suppose you could always ask your employer if they would be willing to pay the SMP as a lump sum - this is an option, although they may have other local policies in place.
Definitely a good idea to watch out for the OMP, if your employer offers it, as KF said. Most companies will require that to be repaid if you do not return to work with them for a certain period of time afterwards. You can always choose to not receive it if you think that will likely be the case.
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