Despite the PC World/Curry's website still referring to Knowhow Movies, it seems that their parent company (DSG) very quickly passed that part of their business onto CinemaNow. As a result, some smart TVs (that could be used to access Knowhow Movies) can no longer be used to receive the service under its new branding:
https://cinemanowuk.zendesk.com/hc/en-us/articles/203161685
CinemaNow has been around (in the USA and elsewhere) since 1999, so it's even more well-established internationally than companies such as Netflix. However it's so new to the UK that they're don't seem to be any reviews available online. Under the KnowHow Movies branding though the service got generally good reviews but, for people who wanted to watch a lot of films, it could work out far dearer than monthly-subscription services (such as Netflix):
http://www.trustedreviews.com/knowhow-movies-review
Assuming that CinemaNow has preserved the UK business model of KnowHow Movies, you get 48-hour access to any movie that you rent, while purchases are for life (but possibly dearer than buying the DVD or Blu-Ray in the shops anyway).