In my experience, it is a waste of time disputing the valuation. The lenders will always err on the side of caution and they will simply take their valuers word for it if there is any dispute over whether the property may or may not be suitable for their mortgage purposes. They are free to take on or decline any mortgage risk they wish without giving any reasons.
You may be able to obtain a copy of the valuation under the Data Protection Act section 7 by making a Subject Access Request but as this is not personal data but related to the property, they may not let you have it. You have to send a fee of £10 with the request and they have 40 days to reply.
If you are determined to dispute the valuation, what you need to do is get another independent valuation which you will have to pay for and submit that to the lender to see whether they change their stance.
Can I clarify anything for you?