HTC hasn't exactly been setting the smartphone world alight these past few years, so it's understandable that a couple of carriers aren't willing to put their chips on the table for the company's latest flagship. O2 and Vodafone have both confirmed they don't plan to range the device, while EE and Three won't let you walk out the door without some form of upfront payment. Contract resellers appear to be more lenient, however. Carphone Warehouse has a particularly attractive Vodafone deal for the power user: £42 per month with no upfront payment for unlimited calls, texts, a 6GB data cap and £50 cashback.
It's fair to say that MVNOs are flat out not interested in the HTC 10 -- not on launch day, anyway. You can pick one up on Carphone Warehouse's iD network (with the £50 cashback offer), but there's little reason to do that when the reseller has better offers on other networks.
If you're looking to buy an HTC 10 outright and be done with it -- and you may well want to, given the complete lack of pay-as-you-go options -- then most retailers ask you to rustle up roughly £570. Clove has the sweetest deal currently at £530, though it doesn't expect to have stock until a week from now. Buymobiles isn't far behind with its £536 offer, but the product page is also sporting a pre-order tag at the moment.
Update: Mobilefun.co.uk is selling the HTC 10 for only £510, with stock of the Carbon Grey model expected to arrive in the coming days.