I don’t think you are foul for saying your 15 year old can be a bit of a shit.
My 12 yr old dd and my husband can be a bit of a shit sometimes, as can I. If they were four I might say “little monkey” or “naughty” but calling an adult or 12 year old a naughty monkey is weird. Bit of a shit works perfectly haha.
As for £20 a week pocket money, some will say £80 a month disposable income is more than they have a month after working every day every week, so it’s way too much for a £15 year old, others will say they give their children £50 a week. IMO it’s about what you cana ford and what you expect pocket money to cover..
My 12 year old doesn’t get any pocket money and knows that the bits she does get adds up to more than her mates get in cash so she wants to keep it that way haha.
I pay
Xbox live at £5.99 a month
Crunchy roll at £4.99 a month
Spotify at £9.99 a month
Phone at £10 a month.
Her dad gives her money for going out with friends to Costa or cinema or whatever.
£20 week to fund extras to my teenage self is a massive amount, but then my mum paid a flat out fee for watching everything on .tv, there was only landline bill to pay and going out with mates involved playing hide and seek etc instead of sitting in Costa... tomtodays teenager I can see how it can be used up very quickly.
If you can afford £20 a week it’s fine, it’ll be less than some others in here give their dc, but also more than others give there’s. It’s what works for your family and her attitude to others maybe nit having same as her that matters in my opinion.