It is hard to answer as an electrician as to what can be done as DIY as if following the regulations (they are not law) then it is hard to see how anyone does DIY as the test equipment is so expensive even if you know how to use it.
However clearly people do DIY other wise there would be no sites like this one. So also it is clear people do break the rules.
I have many times at work given advice and on the odd time I have visited latter and realise the person has not understood my instructions and errors have resulted.
We were given a task write an instruction set to make a cup of tea as if telling a robot so it has no intelligence it does exactly as told. As my son said in real terms it is easy.
Start pressed open kettle filling tap for x seconds.
Switch on heater.
After x seconds switch off heater.
As in real terms we would be using a special kettle like in the old Teasmaid but clearly the idea was to instruct using a stand human type. It is near impossible to consider every option and it is easy to poke holes in the instruction set.
It is the same telling some one how to wire a socket. There are so many variables one is bound to miss something. Now to slowly add to existing knowledge is very different. I may seem crazy but working abroad I have had electricians unable to work out simple ohms law. How they ever became electricians I don't know.
But on another site glad not this one we have a guy who calls himself "Ban All Sheds" as he is so upset about the way the likes of B&Q set stuff to Joe Public. British Law is some times silly they can sell a wood burning stove which it is against the law for you to install in a standard house with no warning that you can't use (keeping legal) what they sell.
TLC a trade electrical supplier has a web site full of instructions
https://www.tlc-direct.co.uk/Technical/index.html links to their huge data base of help files. Not a clue what the firm is like they are not in my area but their data base is really good. There are others like RS components who also have data bases but theirs is more specialist.
Even they make errors I have just noticed they refer to low voltage lighting as 12 volt where in fact that is extra low volts. 230 vac is low voltage.
Anyway using the TLC guide you can learn a lot. But remember an electrician did have a 7 year apprenticeship, by time I came to be one using day release to go to collage it was reduced to 5 years. The government messed things up by raising the school leaving age and clearly by the age of 21 the apprentice needs to be qualified and earning money so now they use a block release system I think around 2 years before they start to work and clearly lacking experience. As a result today the electricians job has been split into commercial, domestic and industrial so to gain all my dad did (born 1926) it would still take 7 years.
As a DIY guy you can't be expected to learn all that. But with maybe the exception of central heating wiring you can learn most of what you need to work in a house.
The law is different for England, Wales and Scotland but in England work in a bathroom and swapping a consumer unit is not allowed without telling the LABC. They say you need to inform them of new circuits, however they don't issue definitions and they say you can fit a FCU and a row of sockets off that which using IET rules would be a new circuit so it is rather woolly and there are arguments galore as to what requires notifying and you would need to read case law to work out the demarcation line and I am no lawyer.
In real terms if the house is yours and you have no lodgers and do not rent it out how would anyone know what you did? Law says anything planned before 2004 is exempt you should see the extent of my plans!
So plug in tester like the EZ150 and in real terms your OK. The EZ150 measures loop impedance the cheaper ones don't which is why I say that model. There are others that also do loop impedance.
The big problem is not asking. People are worried about being caught and don't ask. I have not seen a single court case for an owner occupier. Landlords are taken to court, electricians who are not scheme members are take to court, but not seen a single owner occupier.
Clearly don't upset people. I think it is Part F building regulations say maximum out door lamp without planning permission is 150W why anyone would want a lamp bigger than that I don't know, but it is so easy to see when you have a 500W lamp instead your asking to get caught. It is really some common sense.