I set a personal limit of 30 metres max. on a single cylinder in the UK. Cold water makes freeflows very likely and poor visibility means the risk of buddy separation is increased. I think the deepest I've ever been on a single in the UK is about 26 metres. Any time I have been deeper I have had a pony or twins.
When I did the PADI deep speciality, I had to do the three dives required on different dates, with quite a gap between them due to reasons I won't bore you with. Dive one was in standard jacket style BCD and side slung pony, which I'd used before. By dive two, I'd made a stainless steel backplate and was using it with a single cylinder, small wing and one piece harness. I used this with a 7 litre steel side-slung stage for redundancy on dive 2. Before dive 3, I was booked on a trip to the sound of Mull and knew some of the dives would be in the 30-40 metre range. I knew I didn't like ponies and had always had to borrow one previously, so I went out and bought a larger wing and a set of twinning bands. I had two identical 12 litre 232 bar steel cylinders (roughly the same as a HP100) so I bolted them together and strapped them onto my BP as independent twins.
When I got back from Mull, my cylinders were getting close to an inspection so I had them stick a manifold on and got somebody from the club to check I was doing shut-downs correctly. When I did dive 3 of the deep spec, we were joied by a DM I had not met before. He came up to us at the car to see if we needed any help with our kit. When I hauled the twinset out of the boot, his expression was priceless. He'd only ever dived singles and when I pulled out the 7' hose and asked where it went he had to call the instructor over, who creased up laughing and told me to stop taking the piss out of his DM.
Since then all my UK diving has been on the twinset. If I'm doing an easy quarry dive, I have to put up with the weight to walk across the car park but at the end of the first dive, I just dump it at the side, grab something to eat, and grab it again for the second dive. On a hard boat I find them easier. All the kit is going to the boat anyway so I'm not carrying extra weight. Once on the boat, I get two dives out of it so I don't have to mess about swapping cylinders, while everybody else is fighting for space and struggling to slip a cam-band over a cylinder on a pitching boat.. I have to admit, they are a pain in the arse on a RHIB though.
I do not agree with this theory that they encourage you to push your NDLs. If you need to rely on cylinder volume to stop you exceeding NDLs, you should think about taking up golf. You are taught to plan dives and monitor your instruments on the OW course - if you cannot manage that, you are not competent enough to be diving. The main reason for twins is redundancy. This is essential for techies but there's certainly no harm in having that benefit on recreational dives. If you go down the tech route later, the kit will not feel alien when you do your first course.