First be careful with the words used for power supplies, and what lamps can be powered from what, I have found many LED lamps marked 50 Hz so they are not designed to run on a DC supply, also in theory a driver is a current regulated device, in practice many voltage regulated devices are called drivers, simple transformers give out 50 Hz but electronic transformers likely give out AC power in the megahertz range which limits amount of cable that can be used, also as you seem to realise some, although not all, electronic transformers have a minimum output.
As to 13A socket on lighting this is dependent on where used, in the loft powering the TV signal booster with a notice on it TV signal booster only, it is not really a problem, we were allowed to use non RCD protected sockets when clearly marked for a single piece of equipment like a freezer, so why not use one to supply lights if clearly marked? Well in real terms it is down to what happens when some one puts some thing into the socket which is over the rating.
So if one fits a FCU with a 1A fuse then you have a 13A socket with a wallmart plugged in, should some one plug a kettle in, then the 1A fuse will rupture and no real harm done, however with a 3A fuse then one could find the MCB trips first, and of course with no FCU the MCB will trip. So it is down to a risk assessment, if you loose that circuit what would it mean? So I will look at my house, if the lighting MCB trips, living room OK street light gives enough light, stairs OK the emergency light comes on, rooms to back of the house could be very dark and the way my wife leaves things around yes there is a danger of tripping, garage OK emergency light comes on, loft a problem, but it is floored and the stairs emergency light will show way to exit. So in my house not really too much of a problem, assessing the risk on the boarder line, but as long as marked up, should not be a problem.
However if I remove those emergency lights, then it tips the balance and no risk too high. Had my lights been split up and down stairs then again situation changes, and also if my stairs were not in the centre of house where there is no light from street lights, that would also change the risk. OK theory no 13A socket on a lighting supply, however in practice if you do a risk assessment and you realise no real problem, then use some common sense. However not so sure as an electrician if one can use common sense?
In mothers house we had a door bell powered from a transformer, I swapped the door bell for one needing 12 volt DC or batteries, the social services high lighted a problem when mothers door bell did not work due to flat batteries, so sent an electrician to swap the 8 volt AC transformer for a 12 volt DC power supply, it seemed a simple job. The electrician came and fitted the new transformer, tried door bell and it did not work, investigated and 8 volt AC transformer had been replaced with a 12 volt AC transformer. I highlighted problem, so off the electrician went to find a DC power supply, however only one at his suppliers was one which plugged into a socket, so he fitted a 13A socket to original door bell supply and plugged in the door bell. Now everything worked.
However my mother soon realised there was now a socket under the stairs, so in went the 4 way adapter and now she plugged in the cleaner into that socket, after she put a extension lead into a bucket of water as she thought it was on fire as it had a red neon, I went around with a RCD tester to check why it did not trip the power, I found the new socket under the stairs would not trip when tested with the RCD tester, so I looked for paperwork which I could not find, phoned council asking for paperwork, as they were the people ordering the work, paperwork likely went to them, now I think you can see where this is going, the new socket was not RCD protected, had he simply stuck a label on the socket saying door bell transformer only he could have got away with it. But now also highlighted he had not issued a minor works certificate, so he got into a lot of trouble as not only no RCD or label but also no paperwork except for the bill. So for an electrician to fit a 13A socket to a lighting circuit is rather foolish as he is likely to get into a lot of trouble for doing it. For the house owner to do same, as long as he/she has assessed the risks, and of course today it will need RCD protection then not such an issue.
In real terms having a socket on the lights is no more dangerous than having the lights on same RCD as sockets.
). There is more spare cable/wires than I remember. ...I knew there was enough left to put them both sets on the same transformer if I wanted to... 
