Check your UK Ebay, lesser known authorized Denon dealers often sell new ones on there for very good prices. I've bought all my Denons through authorized dealer's ebay storefronts. Non chain independent dealers will also usually offer lower pricing if you simply call and ask what's the best they can do. The reason I recommend the X3500 is because its room calibration (XT32) is a very significant improvement over the 2600's standard XT. With your mini cube speakers being carried by the subwoofer, XT32's vastly superior bass management/filter count would likely make a profound difference to your overall sound. I should point out though that the X2600 does add Atmos Virtualization, which is a processing feature that attempts to fake Atmos height channels with your standard speakers. I have no experience if (or how well) it actually works in practice however.
Looking on eBay, the only 3500's are from international sellers from the US. It's on the Denon UK website, so wonder how it's just that sold out right now. Just have been a limited release and discontinued early.