EE are in partnership with Apple so I would expect they have more info on what stock they expect to get. Do you actually think that 4 days before release date they have no idea when they will get stock? Unlikely
been pre ordering iPhones via carriers for a while and every year mine used to be delayed (same with a LOT of people - you’ll find many posts on here, on reddit and on carrier forums). Yeah, some handsets are delivered on release date but a lot of then get pushed to unknown dates and it’s just a mess when you call customer services to ask for a date - they just keep lying and giving you different information.
If you want a guaranteed date/estimate buy from Apple.