Short haul and particularly UK domestic routes have very short booking lead times .It is inconceivable that a large number of flights in June i.e. Nearly 6 months away are fully booked...one date because of a large group maybe but not a series of flights.The flights are either not operating or have been closed out possibly pending a decision to cancel or not.
It's also very strange that availability ends mid Month.If it was beginning of a new season or even beginning of a new month might be possible that schedules not finalized ,but mid month ?
Also lack of availability in 2nd half of month will affect bookings on flights which are available earlier in the month as a lot of people looking to book round trips will go elsewhere if they can't book return sector .
Very strange and Clarissa at Easyjet seems completely in the dark !