I agree that £100 doesn't get you far. Depressing, really.
I usually have about £400-500 a month for my "own" spending, but I try to be smart about it. I buy less but better - so for example, we no longer do dinners or lunches at our local pub (which used to be our go-to when we didn't feel like cooking), because the food was nice but nothing outstanding. We'd probably use 3x£35 a month there. Instead, we just have one amazing dinner out a month at a place that's maybe £70, but still works out cheaper than several "ok" meals.
Similarly, I don't pick up cheap tops or jeans or whatever on the high street mindlessly - I plan and save for nicer clothes. I didn't buy anything for months, but then splashed out at the end of September and bought a wool coat. Again, I'd rather have one great thing than lots of passable ones. My skincare items are a bit costly, but I don't experiment much and stick with the things I know work. I have far less clutter in my bathroom than before.
I think it's easy to get caught in trying to get as much as possible for your money, quantity-wise, and thinking, Oh well, that nail varnish is just a fiver, but really it's better to get just the few really nice things you will love with your "personal" money