It varies through the UK what you can do without registering the work, you can do any work in the house, there is nothing you can't do, but some work needs registering with local authority and the cost at in Wales £100 plus vat for first £1000 worth of work means often cheaper to use a scheme member electrician who can register the work far cheaper.
In England very little needs registering, work in bathrooms, new consumer unit and new circuits, in Wales it also includes work outside, and kitchens, the rules were relaxed after the Welsh parliament got control so we are still using the original English rules.
However it does not matter if it needs registering or not, it should still follow BS 7671 current at the time of design, so you should complete a Minor Works certificate, and in the main DIY people don't have the equipment or skill to complete the certificates, so if your work caused some one to be injured you can still end up in court, but there is nothing stopping you from doing any electrical work in the home.
For rented property you need an EICR (electrical installation condition report) or a Minor Works or an Installation certificate to cover all electrical work again varies through the UK, so with rented property you need that paper trail, owner occupied your unlikely to have anyone wanting to view it, so one can get away with not doing the work to the standard required by British Standards, if this was not the case, there would be little point in this forum.
BS 7671 has been undated over the years, and the home must comply with the one in force at the time of design, if you alter the design then it has to comply with the current BS 7671, so if you swap a ceiling rose for a lighting bar with multi lights, you need to decide has that altered the design? If so then it needs to be RCD protected where maybe before the date of design did not require it.
Note it is design not installation date, so where design not changed then still on old BS 7671.
Please remember you have asked if permitted, so you may drive a car with a trailer at 60 MPH on normal roads without the speed cameras catching you for speeding, but the permitted speed is 50 MPH, so we all likely at some point break laws and regulations and get away with it.
So I would fit lights to my own home without worrying about minor works and the like, but that does not mean it is permitted.
