They do 20% off if your lucky with a new account.
I'm already a customer. I've noticed before that they tend to exclude Apple products from most offers in any case.
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I *think* Very's lock and I know CPW's do, everyone else's come and stay unlocked.
No most remain unlocked but those from Carphone Warehouse (and I think PC World/Currys) do lock to the carrier of the first sim inserted.
This is a bit out of date but explains it
https://kenstechtips.com/index.php/apple-iphone-when-sim-free-doesnt-mean-unlocked
I got my iP 6 from Vodafone and got it unlocked quickly and without fuss for free after my contract ended any way, so I'm not worried about it.