You are getting ROYALLY screwed.

You do not need to 'replace the hubs'. Paradoxically 600 GBP is way too cheap for replacing hubs.... They can just drill the bolt and easy out....which is what I suspect they are doing, just charging you 6 times the rate.

They clearly think you have $$ and no clue about cars, so they are just making up fairy tales... 100gbp to get the bolts out (one hour) a few $ for a new bolt. Star tires are just "sold by bmw"...it is ABSOLUTELY fine to have other tires on there, they will cause NO issues. the comment that it will cause 'transfer box issues" is a abject lie. A lie.

if the car is still there, Id walk in and demand to see this "impossible to remove" bolts.

Oh, and when you DO call them on all this, she will either claim (1) you were mistaken or (2) she was mistaken...a thin veneer over a vile practice.

Welcome to the forum. time to protect yourself. They are stealing from you.