There have been periods of time when fee increases were more in the region of 7-10%. This was the case around 12 years ago and went on for several years.
I would work on a basis of around 5%. You need to be sure you can cover it, so err on side if worst case scenario.
Remember to compound when doing your calculations.
Over the course of a secondary education, increasing by close to 50% from start to finish isn’t far off what it can be.
Yes, increases are more cautious now, but rarely under inflation (these Covid years of rebates and temporary freezes are v unusual) and often significantly above inflation.
Open thing to be aware of is the significant increase in employer contribution to teacher pension a couple of years ago....it went up by over 40% and out big pressure on fee bills. Some schools have withdrawn, but do look out for that as not being a positive....tends to be schools with less secure finances and although they save money on pension contributions they are then far less attractive as an employer. Currently less than 15% have withdrawn, so those schools are likely to struggle further to recruit.
Fees are a massive commitment.
Personally I would only commit if you can easily afford it, including the fee hikes (other hikes at key points such as move to KS2 or KS4 behind the standard yearly increases) and extra curricular stuff, without big sacrifices to your lives, which includes your own pension contributions. If you can easily afford it, the great. If it will be a struggle or mean big sacrifices, don’t get into it.