It wasn't quite so much what she did as the way it was done.
The spending plan looked bad and lack of confidence in it caused a technical issue where pension funds had to make it worse to protect themselves from the consequences, which made it even worse and they had to protect themselves from bigger consequences. But this is boring and not why she had to go imho.
The real problem and cause of the whole lack of confidence was that she got rid of senior people and refused to talk to independent government orgs who are there to check the plan isn't shit.
She removed all the arse covering checks the British government has for budgets thinking she knew better, and as is clear with every mad interview she knows nothing.
It owns too much of Africa to be a regional power. It's a superpower even if by a web of alliances/dependants rather than its own military projection