OK, I misunderstood the intent behind your question. I thought you were asking what £50 buys that is worth having. Personally, I think all those gifts I mentioned are worth having and make a good birthday present.
Even if I could afford more, I'm not sure that I would think it was worthwhile buying more stuff for the sake of it. My children live in a nice house, and most of my income goes on the mortgage, council tax, utilities and upkeep. They have all the clothes they need and they are well fed. They don't need endless toys and activities. We have lots of board games already, so we don't play the same ones all the time. What is the purpose of spending more? Is it simply to have more stuff? Because I don't understand that. We've accumulated the toys/games/sports equipment we have over years of getting these things as presents or perhaps buying a new card game to take on holiday.
I feel slightly queasy when I think about all the stuff being manufactured and produced when there is already tons of stuff in the world. It's not just about the cost, for me it's also about waste and the environment and consumerism.
My daughter recently asked about iPads. If she really wants one, then I will probably find a way to make it happen (secondhand, definitely can't fund a new one). But it would be a one-off and probably be her only present from all my side of the family as I'd get my parents and brother to chip in. I wouldn't set out to spend e.g. £150 on presents every birthday because I've decided that a birthday has a value of £150 and anything else is cheap. That just seems daft to me.
Is a day out as a gift only a decent gift if it costs £150? I don't understand the idea that it's the cost of the present that counts more than the enjoyment it brings.
Besides, if it makes you feel better, I did previously comment upthread that I split the cost of a meal out with my ex because the DC like seeing us both on their birthdays, and I also pay my half of (and usually host) parties if they have them. So my total spend is more like £150 per birthday.