We're talking about new Samsung flagships here, so it's no surprise absolutely everyone is hoping for a piece of the action. You'll obviously have to look at what allowances each of the four major carriers are offering with their plans, but it's probably best to start your search at contract resellers. Unlike the carriers themselves, they have tariff options that don't require any kind of upfront payment, as well as plenty of reasonably priced contracts not available elsewhere.
As usual, MVNO Tesco Mobile is offering some seriously competitive plans, starting at £37.50 per month with a 2GB data allowance and no upfront payment. giffgaff's pricing is typically quite reasonable, too, but it appears to be having some stock issues at time of writing. It's almost not worth mentioning other players, but Virgin Mobile's tariffs start at £34 per month for a 250MB data cap, and TalkTalk Mobile wants £37.50 per month from existing customers for a similarly small allowance (both of these options come with no upfront costs). It's best to avoid Carphone Warehouse's iD network, with the basic 1GB plan requiring £80 down and £37.50 per month.
Since you can't get the Galaxy S7 on pay-as-you-go anywhere, buying an unlocked model is currently the only way to escape a lengthy contract. Unfortunately, though, there's no one retailer we can point you towards for that killer deal. All the usual suspects (Expansys, Unlocked Mobiles, Clove, Amazon, Tesco Direct and Samsung, in this case) are selling the S7 for between £550 and £570, making Mobile Phones Direct your best option with its £540 price tag.
Galaxy S7 Edge
The Galaxy S7 Edge is the more expensive of Samsung's new smartphones, with a bigger screen and curved glass edges. Like the S7, it's available at all major carriers and resellers, though again, it's the resellers that have some of the best deals, with several options that don't demand an upfront payment. You aren't able to get the S7 Edge on pay-as-you-go anywhere, by the way.
giffgaff doesn't seem to be having a great deal of luck with stock at the moment, making Tesco Mobile the clear MVNO favourite with plans starting at £41 per month (nothing upfront) for a 2GB data allowance. Virgin Mobile's tariffs aren't anything to write home about, starting at £38 per month with no upfront cost for a 250MB data cap. Carphone Warehouse's iD network, though, is the worst of the bunch. Its 1GB plan is £37.50 per month, but also requires a £130 down-payment.
If it's a SIM-free phone you're after, then head to Buymobiles, Unlocked Mobiles or Mobile Phones Direct. They're all offering the S7 Edge for £620, which is the lowest price we can track down right now. Sure, you're not exactly getting a bargain -- £630-£640 is the going rate at Expansys, Clove, Amazon, Tesco Direct and Samsung's online store -- but what you save is worth a couple of pints, at least.