Research into the now-defunct Sky Games platform, specificity the Digibox portion.
Information is just listed chronologically as I (or Dan) thought of it. Maybe soon I'll have a go at neatening it up.
11/04/19 - JaffaMeme: I have currently contacted Thomas Kronberg in an effort to learn what language the games used (He's the man who worked on quite a few of the sky games). I believe they used Java but wanted confirmation or correction directly from a sky games developer. You can look at the thread Here.
12/04/19: Dan's Tweet got a response confirming the games made by Denki were written in C. The dev later mentioned that the games' turnaround time was often only 4-6 weeks!
Sky boxes and their interactive services ("Sky Active") were based on “OpenTV” which presently seems to use HTML5 and essentially be Electron for set-top boxes. Perhaps at the time these were just glorified Flash games as Dan thinks is plausible?
Despite Dan's memory, a landline was not required; we paid for the various passes through my mum's feature phone credit.
The games portion was formerly called Sky Gamestar.
While also referencing OpenTV and the Sky Boxes, the game development companies also mention DirecTV, a similar satellite telly service operated in the States.
According to Wikipedia:
The set-top box middleware is provided by OpenTV, but the EPG and all the software extensions that manage the PVR functions are produced by NDS under the name of XTV PVR
All Digiboxes used to run on OpenTV with Sky's EPG software and NDS VideoGuard conditional access. The Digibox receives software updates over the air, even when in standby mode should an update be available. The software features [...] the ability to view OpenTV or WapTV applications provided as "interactive" or "teletext" content on channels.
It certainly looks like the Sky interactive services used WapTV, a markup language/microbrowser for set-top boxes. The Wikipedia page directly calls out Sky's use of the system:
The platform brings internet-style content and interactivity to the Sky Digital platform by deploying a purpose-built WML microbrowser to the Sky Digital set-top-box over the satellite broadcast stream. Web site owners simply have to serve well formatted WTVML from their web servers to allow Sky set-top boxes and consumers to fully interact with their web services. In March 2007, Sky together with some of the Waptv founders "spun off" the technology into a new Company "Miniweb", with the goals of furthering the work on WTVML in a global forum, and enabling the deployment of the system in other networks and devices
Here is an archive of Sky's WapTV developer programme homepage. Sadly most of the content is locked behind the forum's authwall.
The closest match I can find is the Amstrad DX100, although I swore the thing was grey.