Basically the scam scheme is this.
For normal companies:
sales £400m (about what Starbucks did in the UK)
Rent, wages, milk, coffee, rates, maybe £320m
Profit £80m
Corporation tax due: 26% on £80m = £20.8m
Except for Starbucks, the £80m profit is paid to an offshore company owned by, er Starbucks, as a 'licensing fee'. So their profit is deemed to be zero in the UK. The £80m may be taxable offshore, but probably not, and even if it is, then the UK still don't get anything.
So another reason to avoid Starbucks.
Beside the shit coffee that is.