This is relevant with regards to ABTA I think (from the link above)
"Complaints we can’t help with
Complaints that are not about ABTA Members
If you’ve booked your travel arrangements with a non-ABTA Member, we can’t help you. We govern our Members through the ABTA Code of Conduct, this is something all of our Members sign up to, but we have no governance over non-ABTA Members.
You can try contacting Which? or the Citizen’s Advice Bureau with your complaint.
ABTA Members are travel agents and tour operators. Not, for example, hotels or airlines.
Check your confirmation invoice to see who your booking is with. An ABTA travel agent can still book you with a supplier that isn’t an ABTA Member. This often occurs when you’ve booked a Flight-Plus travel arrangement. Flight-Plus travel arrangements are not package holidays.
With a Flight-Plus, your travel agent has placed you in a number of contracts with the suppliers of your travel arrangements and this will be clear on your holiday confirmation invoice. Your travel agent’s terms of business will explain that they accept no responsibility for the acts and/or omissions of the third party suppliers that they have booked you with.
Those suppliers may not be an ABTA Member. If you have a complaint about one of those services provided, if the supplier is not an ABTA Member we cannot help you. "