Tesco give great low prices and deals on the Ninja Foodi and often the best price on the Ninja Foodi Slow Cooker, the Ninja Foodi Deluxe and great prices on Black Friday and all year round in the UK. Amazon are a perfect choice for low prices for the Ninja Foodi Pressure Cooker and also the Ninja Foodi Deluxe.
The Ninja Foodi at Costco is probably the best place for the best deals and lowest prices but getting a Ninja Foodi at Currys is also a sure way to get the lowest prices.
So in summary for the cheapest Ninja Foodi in the UK try the following stores in this order. Amazon for the cheapest deals delivered, Costco if you're a member and can pick it up locally, Currys for good prices on the Ninja Foodi deluxe and Argos for just great ranges and prices on the pressure cooker and deluxe versions.