Upgrade programme, do I have to make a store reservation first?

Hello,

I'm in the UK and want to buy an iPhone 7+ through the Apple Upgrade Programme. On the Apple UK website it implies that I have to 'Reserve an iPhone to buy in store' however each time I check availability to reserve a phone it says there aren't any Phone 7+ available, only the iPhone 7.

Can I just go into an Apple store and buy the 7+ there-and-then through the Upgrade Programme without a 'reservation' or do I always have to make a reservation at the store first?

http://www.apple.com/uk/shop/iphone/iphone-upgrade-program

https://reserve-gb.apple.com/GB/en_GB/reserve/iPhone?execution=e2s1

Thanks.