Hello

I don't know of any City and Guilds courses that can be taken at home, because it just isn't possible to undertake the practical side unless you can attend either college or a training institution.
The NVQ courses also involve an element of workplace assessment from the college, as well as the inclusion of a brazing and electrics assessment.
Some of the best courses I've done have been run by the various manufacturers, depending on whether it's air con or refrigeration you are looking for. The VRV and VRF courses in particular are very informative.