Much of the OSC's focus is on the fact that users of other desktop operating systems are not able to access the service. The BBC points out that this still means that the majority can. While this may be true, it effectively excludes some categories of devices from supporting this service.
The Home Theater PC is a relatively new innovation. Until recently, storage and processing costs made the concept too expensive for most people. Microsoft even released a "Media Center Edition" of Windows to run on them. While Windows MCE is popular on a HTPC systems that are also intended for general purpose computing, it is by no means ubiquitous on special-purpose devices. Manufacturers building consumer-electronics type devices for this role often prefer Free Software, since it allows them to make any modifications they need to, and doesn't require a per-device license fee. Tivo is a good example of this, although not one popular with the Free Software community.
When designing such a system for the UK market in future, OEMs will have to weigh the benefits of Free Software against the fact that they can only provide iPlayer support if they use Windows. Since a device without iPlayer support is likely to sell less well than one with, they are likely to choose Windows, irrespective of other benefits.
One manufacturer likely to be affected by this is Apple. Their recently-launched Apple TV plays videos bought from their iTunes store and downloaded from YouTube, but it will be locked out of supporting the iPlayer service since it runs OS X. Microsoft's CODECs have been ported to OS X, but their DRM remains a Windows-only product.
When digital TV was deployed in the UK, it was based on the DVB-T standard. This standard can be implemented by anyone. The GNU Radio project has a pure software implementation under development, to complement their existing ATSC decoder. People with existing TVs were required to buy an external DVB-T decoder box, but they could do this from a number of different suppliers. The situation with Microsoft's DRM is very different. There is only one suppler: Microsoft. I suspect that there would have been some serious complaints if the BBC had deployed a form of digital TV that only worked with Sony TVs.