It all depends on your income and budget, surely? I put some money aside every month to cover birthdays, so have a fairly fixed budget.
We go out for a meal on the DC's birthday. I'm divorced, but grin and bear a meal out with ex twice a year for the DC's sake so that they can see us both on their birthdays. So that is probably about £100 per birthday (£50 for my share). We split the cost of parties, but I tend to organise/host. DS didn't really do anything for his last birthday (teenager). DD has often had sleepovers, so the cost is quite low. Obviously more if they do an activity with friends. I don't think we'd spend more than £100 on a party, though (£50 per parent again).
When they were younger, we did parties at home and they each had one soft play party in primary. But mostly we did it ourselves with traditional party games, some crafts etc.
So, per child that's £200 if family meal + party with paid activity. Or around £130 if it's family meal + sleepover/party at home.
I make the cakes myself. My budget for presents is about £50 each (which is about what we spent before the divorce. Ex was tight about presents).
Around age 6, I'd say we spent a maximum of £250 per child per birthday for everything (probably be £300 now).
They're older now, so parties are smaller affairs with a handful of close friends.
If they want a specific present that's over my budget then I ask my parents or brother to contribute. Try to go for experiences over stuff if possible, but maybe get something else to actually open, e.g. a book and/or item of clothing. But I usually buy secondhand to reduce cost and for environmental reasons. DC are fine with that. If possible I get new with tags for birthdays (in the sales or from Vinted).
I can't imagine £300 worth of presents every birthday. Maybe as a one-off if they wanted a high-value item like a games console, and even then I'd probably ask my family to contribute and would have to cut back on other things for a month or two to pay for it. But if you can afford it then does it matter what the actual figure is?