Hi....so long as the rental will cover the mortgage payment and you are happy that there will be no vacant periods then affordability may not be an issue. However the new lender may well want to see tenancy agreements set up and the property rented, unless the new mortgage is a "let to buy" product. This will allow a second purchase based on the rental assessment of the current property without proof of rental??? I stress these points because applications have a funny way of going "t*ts up" when the underwriter gets hold of the case and starts to ask questions.
On the point of further borrowing I would wait until the mortgage has completed to ensure a smooth completion. If HSBC are happy to consent to lease, should you tell them, and there is enough equity to assure a second charge then this may be less expensive initially, although the secured loan will be difficult to place on a rented property. You see the difficulty in advising unless one is completely aware of all circumstances. I would wait until the purchase goes through and then see how the land lies, but I do see problems ahead I'm afraid.
Regards
Ian