The test is whether you get an ATOL certificate when you book.
From the CAA site:
Does ATOL cover bookings made online?
Yes, as long as your holiday is one for which protection is legally required and you are booking with a UK travel company.
For instance, if you use a website's 'flights and hotel' booking option to purchase flights and accommodation at the same time, then the law says your travel company must provide ATOL protection.
However, ATOL only covers bookings made with travel companies, not those made directly with airlines.