For the UK you are required to have your level 3 tech' cert' and 17th update and to certify your own work. To test other installs you are required to also have the 2394 / 2395 inspection and testing certs. As for ECA, NICEIC, NAPPIT etc they help the process but technically you don't have to be a member but it does make dealing with building control a lot less hassle. I have some colleagues who have given copies of all their qualifications to building control and conducted rewires in that manner but they have to pre-warn what they intend then have building control come out and inspect it then you pay for the privillage it makes sense if you only vary rarely conduct electrical installation work. When it comes to site work you need all of the above plus you also need level 3 NVQ and ECS card to be considered a fully qualified electrician. Plenty of hoops to jump through I'm sure the rules are slightly different for Scotland just to make things more complicated!
I would have thought if you are working mainly in Scotland that Select would be a good choice.
Clear as mud that's how they like it.