afaict there is no official guidence from the IEE.
Most sparkies have gone for the convention introduced by the NICEIC which makes sense from some points of view (makes joining cables and repurposing cables with burried joints in simpler) but is more likely to be confusing to the diyer and therefore IMO a mistake for domestic installations.
ultimately whichever you choose to do make sure you sleeve it appropriately
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local