ATOL regulations may have no standing in the IOM, but as you are selling tickets to UK passengers for travel originating in the UK, you are bound by the regulations for any such sales. The ATOL regulations do not apply only to packages - they apply to seat-only services as well.
If you are acting as a ticket agent on behalf of an AOC holder, you are required to make that clear. I am sure that the CAA would have told you that. I cannot imagine that they have sanctioned an arrangement where there is an obvious absence of transparency, especially given the sensitivity around this. It is a legal obligation that the passenger must be aware of the party with whom they are entering into a contract for carriage. The ticket agent arrangement also means that if Ettyl was to cease trading, the AOC holder (so Eastern) would have an obligation to either carry the passenger or refund their money if they elected not to fly (and pay compensation depending on the notice given of flight cancellation).
Let's cut to the chase:
Question 1
If I buy a ticket from you, with whom am I entering into a contract?
Question 2
Does your agreement to charter the aircraft from the AOC holder either include an agreement for you to act as a ticketing agent on the operator's behalf or is there a separate such agreement?
Question 3
Have you actually received UK CAA acceptance of this or any other arrangement which confirms you are exempt from the need to hold an ATOL?