A Single Joint Expert is that an expert...if valuing a property then an expert is usually regarded as someone who is on the RICS Surveyor's register. If the valuer in not on the list then you could validly argue for instructing someone on the list.
I don't see a problem with the valuation, it is being made on the assumption of a market remaining constant. They have to base their value on something, and the very reason for doing this is to protect you and themselves. If the market does change then you have the right to get another valuation based upon a volitile or drop in the market. Unfortunately, they do not have a crystal ball and can only value on this basis,
It is not an ideal market however, I would suggest that you do get advice on settlement when you reach that stage, most importantly because there is more risk with the current world situation when settling or the court making final orders, but there is no getting around this. No one has a crystal ball.