The following is a fairly comprehensive and up to date guide on the subject of whether iPhones will be locked or not taken from the article linked last night by Jack1998:
http://kenstechtips.com/index.php/apple-iphone-when-sim-free-doesnt-mean-unlocked
Retailer: Will the iPhone be permanently unlocked?
Direct
Apple Store ✔ Yes, the iPhone will be permanently unlocked.
Mobile Network
giffgaff ✔ Yes, the iPhone will be permanently unlocked.
Three ✔ Yes, the iPhone will be permanently unlocked.
EE ✘ No, the iPhone will be locked to EE.
O2 ✘ No, the iPhone will be locked to O2.
Tesco Mobile ✘ No, the iPhone will be locked to Tesco Mobile.
Virgin Mobile ✘ No, the iPhone will be locked to Virgin Mobile.
Vodafone ✘ No, the iPhone will be locked to Vodafone.
Third-Party Retailers
Argos ✘ No, the iPhone will lock itself to the first network.
Carphone Warehouse ✘ No, the iPhone will lock itself to the first network.
Curry ✘ No, the iPhone will lock itself to the first network.
John Lewis ✘ No, the iPhone will lock itself to the first network.
PC World ✘ No, the iPhone will lock itself to the first network.
Just looked at the part numbers used by Apple and what the other retailer are using and they are exactly the same as Apple, so hardware wise they are the same anything around "this will lock on the first use" is BS and even if it isn't is only ~£10 to remove a lock, so dont worry about it...
Apple iPhone 7 Plus 256Gb - Black
Direct Apple Part Number:
Part number: MN4W2B/A
Very.co.uk
MPN: MN4W2B/A
EAN: 190198045348
Argos:
EAN: 190198045348