In addition to Microsoft's own online store in the UK and Carphone Warehouse, residents in that country will be able to get the Lumia 950 from Vodafone and EE, according to posts on their official Twitter accounts.
In addition, Vodafone has also confirmed it will sell the budget-priced Lumia 550 as well. Neither carrier have offered any more details on their plans for Microsoft's new phones, including pricing and availability. Also, neither carrier have announced plans to offer the larger Lumia 950 XL for sale.
Thanks to Nicholas for the tip!