As I understand it, the only actual requirement under the building regs, in relation to electrical work is as follows:
Reasonable provision shall be made in the design and installation of electrical installations in order to protect persons operating, maintaining or altering the installations from fire or injury.
This is in addition to the general requirment that all work shall be carried out with adequate and proper materials and in a workmanlike manner.
The difficulty comes where the work needs to be approved by the local authority. Such approval is
not required to add light fittings and switches to an existing circuit or to add socket outlets and fused spurs to an existing ring or radial circuit
unless the work is in a kitchen or special location (basically where water is involved) or to a special installation (various odd things, none of which are likely to involve home cinemas).
What many people consider to be part building regs are in fact approved documents issued by the relevant Secretary of State. They are said to be for guidance only. Clearly, where there is a requirement to submit plans to the local authority, it would be difficult to get approval for something which is not in accordance with the approved documents. However, for work which is exempt from involving the local authority, what one does and does not have to do is more of a judgement call.
The relevant approved document can be found
here. This in turn makes reference to various BS and similar standards and regs.
The rules about wiring are summarised in the diagram on page 37. But in essence, if I understand it properly, vex is correct. Wires can run horizontally or vertically in an imaginary channel the same width as the wiring accessory. Accordingly, diagonal is the no-no.
It follows, therefore, that the sky man who drilled the hole where he did was a Muppet and there was nothing wrong with what the house builders did. Although, I still think it ought not to have been necessary to have a spur in a new build house.