The general rules for companies in VAT groups are that:
– the entities involved are body corporates, under “common control”
– the entities must be “established” or have a “place of establishment” in the UK
Note as you pointed out, and for the purposes of Group VAT regisration Isle of Man is treated as part of the UK and hence the mix of Isle of Man and UK is not an issue.
This is worth a read if you are considering a group vat registration: https://www.gov.uk/government/publications/vat-notice-7002-group-and-divisional-registration/vat-notice-7002-group-and-divisional-registration–2