"how can England not be subsidising Scotland"

Different points really. Scotland does have higher spending per head than England does. It naturally would as spending throughout the UK depends and is worked out on the supposed cost of services to a particular area. That applies within England too. Scottish spending is over and above what it should be even on that basis. However that isn't the same as being subsidised by the rest of the UK. Whether we are subsidised or not depends on how much Scotland contributes compared with how much we take out and not on how much is spent per head!