You can get ERF seats cheaply now. The Joie Tilt for example is £75 which is cheaper than a lot of forward facing seats and you can buy it in Mothercare. That will rear face most children until 3 or 4 years. When I was looking for DS1 you could only buy them online or at specialist retailers and they were all about £3-400.
DS1 was 13 months in his dad's car (we'd split up so he made the decision) and 18 months for me when he'd outgrown the baby seat. This was 10 years ago and it was really hard to find ERF seats which were affordable and worked for a non driver. I got a Kiddy seat with impact shield which at the time were said to be an in between measure safety wise.
It's not just the UK that people FF early, it's also done in other European countries like France and Spain. Like here you can get ERF seats but they are harder to find.
The 15 month rule is not actually law, it is legislation which means it affects products being produced and sold, not the behaviour of parents who have bought seats produced under the older laws (which are still legal to produce and sell).
DS2 is only 3 weeks old so will be RF for a long time yet but I'm not sure what we'll do. Personally I think the evidence for RF is extremely strong until around a year of age so I would want to RF past a year, probably to 18 months minimum to be safe. Apparently across Europe when the only seats available stuck to the 9kg rule, there was a spike in child passenger road deaths at around a year of age and this was found to be attributed to children tending to be moved into forward facing seats around the end of the first year.
After two years although rear facing is still safer (it's always safer) you're not talking stark life or death any more so after that point IMO the safety is weighted as a bit less important, and other factors such as travel sickness, behavioural issues (escaping belt), social contact, space in car, cost of seats, ease to do up harness etc would all start to factor in more heavily to my personal decision making.