Hi Adam,
Only the Sony RDR-HXD870 will allow you to control an external set-top-box using its G-Link feature and Guide Plus+. The unit comes with a special infra-red set-top-box controller that you plug into the back of the DVD recorder and place on top of the external STB. You can then change channels on the set-top-box using the recorder's remote, plus the recorder will automatically change channels on the STB when making a timer recording using the Guide Plus+ EPG.
I don't think you can browse the Virgin box's own EPG through the Sony, I think you're limited to the Guide Plus+ EPG. The list of controllable set-top-boxes is constantly updated by Guide Plus+ and it supports all major brands of external satellite, cable and Freeview receivers, so Virgin should be on there. The trouble is, Guide Plus+ is a real pain to use, so be warned.
Some of the others DVD/HDD decks can be triggered to turn on and start recording by an external STB when connected via Scart. The Panasonic's EXT LINK and Toshiba's Satellite Link both allow you to do this. In both cases the external STB needs to be equipped with some sort of timer, which will turn itself on, activate the recorder and start it recording, then shut it all down when the recording has finished. But neither recorder will actually allow you to control the Virgin box, they're merely 'slaves'. The Samsung doesn't offer this feature.
And Peter, yes the V+ box does record subtitles, and you can switch them on or off as required for playback. But if you archive recordings from the V+ hard-disk onto an external DVD recorder or VCR (like the four I tested here), the subtitles won't get recorded.
Hope this helps,
Danny