I was mulling on the availabilty of players at the start of next season and dug this out from the RFU site :
From 2019/20, the Premiership will start at least one week later in September for the next three seasons, with the final taking place over the last two weeks of June before the summer tests kick-off in July - the one exception being next year when four rounds of Premiership Rugby Cup are played alongside the Rugby World Cup - starting on September 20 - before the league begins on 20 October.
So JTA (and any other potential SH imports) will get a break after S15 and reasonable preseason.
It will be interesting to see the Heiny schedules for next season. Possibly first pool round in December.
RWC is September 20 until November 2.