OP if the Sky iPad price is better than the Apple price by a meaningful amount, there is probably some "lock" so they get at least the difference- if not more- back. If the price is about the same as Apple- give or take a small percentage- it is probably unlocked... like buying one directly from Apple.
If in doubt, actually read the fine print and/or ask Sky (and then confirm if they say "unlocked" by reading the fine print).
The objective for every seller will be to get close to the Apple price- give or take- for each unit sale. Any exceptional bait vs. Apple pricing probably has a hidden hook in it. See "free*" iPhones vs. the unlocked price of iPhone on the Apple website.