Ther should have been a link in the email to a page on the easyJet website giving more details. If not, this is the link:
https://www.easyjet.com/en/policy/partner-airlines#wdl
Airlines (and travel agents) in the EU have to disclose the operating carrier of every flight, when offering it for sale, when it’s changed and before the doors are closed on board the aircraft.
It’s the reason for the pre-departure flight deck announcement along the lines of “welcome on board this British Airways
operated flight to Paris Charles de Gaulle.”
The wording is very specific, and the timing is too, as a passenger has the legal right to cancel/offload and a refund if the operating carrier is not the one that was disclosed at time of booking or prior to boarding.
In the easyJet scenario here, Berlin Tegel is a new station for the airline, and the story is that they have slots available to use before they have enough of their own aircraft to operate the services.
For a few months until the end of the Summer, they’re therefore chartering in some aircraft and crews from Thomas Cook/Condor, SmartLynx and WDL. In fact, SmartLynx also do a lot of so-called wet-leasing to Thomas Cook, and I think some SmartLynx aircraft carry Thomas Cook branding.
If I were to guess, I’d say that for a Gatwick flight, SmartLynx would be the most likely carrier, and you should receive a further email from easyJet confirming the actual ‘Air Operator Certificate’ holder closer to the time.