Interesting. I'm not 100% sure of the answer - I suspect you won't know exactly unless you approach a lender with regard to the mortgage on your new property and disclose to them all the relevant facts. It will also be the case that different lenders will have a different approach.
I suspect if you maintain your current mortgage and simply get a consent to let, then a new lender will probably want to take that loan into consideration. Your income would need to be sufficient to service both loans, so enough for £533,000 not £400,000. If the current mortgage were changed (by re-mortgaging) to a 'proper' buy to let loan, then your new lender probably wouldn't take it into account. Some lenders give consent to let in certain temporary circumstances i.e. you are moving away because of work - but there is a distinction and your current lender might be unhappy if your long term intentions were to hold the property as a buy to let investment, rather than selling it to clear the loan.
It is difficult to give a definitive answer.