You can get cheaper RF seats. The cheapest is Joie tilt which will rear face until 18kg which is about 3-4 years and costs between £50-70 depending on whether it's on offer.
Even £200 for an isofix or longer lasting 25kg (6-7 years) one is actually comparable to the cost of a decent FF isofix seat anyway. And if you're on a very tight budget the best way to get safety for cheap is to rear face anyway. None of the sub £100 forward facing seats are rated that well whereas it could be the oldest most basic RF seat and protect the head and neck well simply because of the design of it.
Anyway in terms of age. I had planned to go to 3 but communicated to DH that I was OK with DS2 forward facing on occasion, as we have a spin seat, like if he was totally struggling to get into the seat and he needed a novelty, or if he was alone with him and wanted to keep an eye on whether he was eating or sleeping. Then I started a new job, DH interpreted my words as "it's fine for him to FF now" and as he was the one with DS in the car mostly, put him FF all the time even though he knows it's not as safe so we had a compromise for a while where we would put him RF if we went on a motorway, but then there was always a reason this couldn't happen as well, and now DS2 hates rear facing so I gave up and bought a better FF seat. He's now 2.8.
Next baby is due in summer as well so I will say 2.5 as a minimum to get through winter as I was always anxious about that. Then I'll probably not worry about it.
I think ERF is important to be aware of, and would always try to help people with the info to do it as long as they want to, but the difference between FF and RF does get smaller the older the child is. These are the milestones I've found in my research:
Able to sit unaided for 20 mins (6-10 months, 9kg) - old milestone back from when car seats were first invented. Not recommended any more. Babies this young are at high risk of internal decapitation in a crash FF.
15 months - minimum for isize seats, also the place where if you want to prevent the most deaths caused by the higher risk of FF, you draw a line here and there it is. Again barest minimum, would recommend longer if possible. FF seats reduce risk of injury or death by about 60% whereas RF is about 92% - hence the 5x safer statistic. Its actually 5x more risky to FF at age 1. 8% vs 40%. 5x safer doesn't work, it's more like 1.5x safer because you're counting a different way. An easier to understand context is if 25 families had an accident severe enough for anyone to be injured, 4 1yos will be seriously killed or injured either way. 30 will be fine or only minor injuries. In the middle are 16 which would sustain serious possibly fatal injury if FF whereas only minor or no injury when RF.
Age 2 - US/AAP recommendations fall here. By this age, the difference has fallen a lot more. It's more like 15% difference rather than 150% or even 500%, but I couldn't find where this is calculated from. Even if you assume the worst case scenerio though that figure gives you 92% injury reduction for RF and 78% for FF so if we go back to our 50 families, again 4x 2yos will be killed or seriously injured either way. 39 will be fine in any properly fitted seat. Now there are 7 in the middle, who would be fine if RF but not fine FF.
3 (somewhere between 3.3 and 3.9, prob 18kg approx) - when statistically most children in Sweden are turned FF, Sweden being the country with the lowest child passenger road deaths in the world and a vision that nobody dies in a car. Their stats are very very low.
Age 4 - where all the European studies say to RF to, including the British medical journal. Interestingly the max age to rear face allowed in Australia. All Australian seats use top tether and apparently they have never had a FF child die in a top tethered seat. They harness much longer than us and delay moving to a booster seat.
Age 6 / 25kg - 25kg is the max capacity of the most easily available RF seats. Age 6 is when the neck vertebrate have solidified so a child's spine is equivalent to an adult, they would from this age tend to get whiplash rather than spinal cord severing unless an impact is very severe. The evidence also shows children aged 6+ are less likely to be injured in a high backed booster than younger children.