Like matana, my view would be to spend only what we can afford, and to treat all the kids "equally".
I use quotation marks there, because I don't think it is a matter of spending the same amount for each kid on each and every birthday - my DSD just turned 18, and my DD is turning 3. I would never spend as much on a toddler's birthday as a teenager's anyway, and since 18 is a particularly special birthday, we definitely spent quite a bit more on DSD this year. But it is important to me that when DSS turns 18 in a few years, he gets the same sort of 'big deal'. Or when DD turns 18 in fifteen years!
Of course, things change, and kids have to understand that, too. If we are struggling more financially when DSS turns 18, he may not get the same lavish treatment as DSD has just had. I'll do my best to make sure that he doesn't see it - trying to get the same bang for less buck - but the reality is that we can't over-extend ourselves for a birthday.
As for once the kids become adults - yeah. I expect the budget to come down. If we came into an unexpected windfall, we'd buy nice gifts for the kids, but the expenses of a childhood birthday - throwing a party, toys and gifts - are over once they become ostensibly "independent" young adults. That isn't to say they won't be getting gifts - but I think the budget will be smaller.
My parents definitely stopped giving 'big ticket' items after we moved out/went to uni. They still give us presents, but they would be on a smaller scale. That changed right away after 18, but even more so after they retired. They are living on a fixed budget themselves now - we spend more on their gifts than they do on our's. That's only right.
I can't really say whether £100 is too much in your situation, because to me it would totally depend on whether you can easily afford it or how much of your income it represents. We spent what amounted to two weeks of my wages on DSD's birthday, and I do think it was too much, as we are not in a position of financial security.