A new Roku, the Roku 4 ( with 4K HD) has been on sale in the USA for quite some time now, as has a new Roku 3 with voice search, a new Roku stick is also available, however there's no signs of nay of these products making to the UK.
This topic was started over 12 months ago ( as of 24th April 2016) and the new Roku 3 has still not materialised in the UK.
The only conclusion is that Roku are either dumping old stock in the UK, or outside of Now Tv boxes, Roku have no interest in providing their streaming boxes/sticks in the UK, Stockists such as Currys PC world dont appear to have much in the way of Roku boxes for sale, just empty display spaces taken up with Now Tv boxes, and Sling Boxes