With the whole DAZN - BT thing I wouldnt be surprised to see Joshua fights on DAZN worldwide but on BT sport PPV in the UK.
If BT sell the BT sport business to DAZN, BT sport will no longer exist. There is not a chance DAZN would not rebrand.
I have seen ads for the upcoming fight being on DAZN - is the fight only on DAZN outside the UK - I assume so as opposed to multiple platforms - which makes me think its likely the present arrangement will continue - Sky PPV in UK - (or highest bidder) - DAZN everywhere else