My understanding is that when a formerly non-UK resident trust migrates into the UK, the stockpiles of relevant income, OIG and other capital gains continue and operate, until they are exhausted, in the same way they did when the trust was non-UK resident. Obviously nothing will be added to those pools once the trust is within the UK tax net.
Was the undistributed accumulated income capitalised once the life-interests had all been attained? In any event any ‘capital payments’ (including actual and deemed e.g. interest-free loans and/or use of trust assets rent-free) would crystallise those pools in the hands of the relevant beneficiary in the set-off order of relevant income, OIGs then other gains - potentially also with a surcharge applying, depending on the specific circumstances.