SDLT is charged in slices: nothing to £125,000, then 2%, 5%, 10% and 12%. A non-resident buyer adds 2% on the whole price, and an additional dwelling adds a further 5%. On £500,000 the standard bill is £15,000 before either surcharge.
Most London flats are leasehold. The remaining term, the ground rent and the service charge decide the price as much as the postcode does, and below roughly eighty years an extension starts costing real money that grows every year you wait.
Since 2017 cladding and the EWS1 form have repriced whole buildings, and lenders treat the building safety pack as underwriting. Ask for it before the offer, not during the mortgage application.