understand the value to a landlord is the rent receivable, from whom and for how long.
The value for you is as an owner occupier. These are not the same thing and rarely come to the same numerical value.
The office coming vacant upstairs is a problem and a blessing. It’s a problem in that there is not a core chunk of income on which the lending bank can hang their loan, a blessing in that it will suppress the price of the property.
You really need professional (MRICS) advice before putting a figure on anything.
The bank may want you to put a lease in place on your retail space. They may accept that some other personal guarantee may suffice, or even cross-collateralisation with your house.
Whatever, you may find you have little access to the right/flexible/best value property loans without using a specialist broker.