None. The Sewel Convention is not relevant to the UK’s decision to end the reciprocal health care agreement with the Isle of Man, as it involves neither a devolved legislature nor any form of legislation.
The Sewel Convention is relevant to the relationship between the UK Parliament and the Scottish Parliament. Similar conventions apply to the relationship between the UK Parliament and the National Assembly for Wales and the Northern Ireland Assembly. The convention provides that the UK Parliament would not normally legislate with regard to devolved matters except with the agreement of the devolved legislature.