I don't have a John Lewis near me, so not sure if they do actually price match, as technically this a cash back offer, not a discount. Curry's aren't the greatest and most reliable company in the world, but they had this cash back offer a few years ago when I got my iPad. With most things, as long as nothing goes wrong (which doesn't seem the case with all these MacBook Pro reports) you'll be okay. You can still buy from Curry's and take it into the Apple Store with a problem, just can't return it there incase you don't like it, which Curry's doesn't allow if its opened, I think.
Their margins aren't great, I imagine. Even though 10% back is a nice chunk, you'll have to keep in mind these cash back offers are always a bit convoluted. You buy the MacBook now, and it gives you a time window to go claim the cash back. When I got my iPad, it was 2 weeks in the month after I had to claim it. People forget, lose their receipt etc, so cannot claim the cash back. This claws back some of their margin.
I personally am skipping this generation anyway, but for people who are on the edge about the price, especially with the UK, and don't mind shopping with Curry's, this is a great offer. It at least absorbs the price increase over last gen.