The problem you have is that, unless you jointly elected for one of the two properties that you own to be treated as your main residence within two years of having acquired the second, the decision as to which is your main residence is based on the facts and, to a certain extent it is out of your hands and in the hands of the tax office. A married couple who are living together (marriage not broken down) cannot have more than one main residence between them at any one time.
Clearly, up to the point that you acquired the second property, the Surrey property was your main home. If you sell the Dorset property, unless you can convince the tax office that the Dorset property was your main home from 17 years ago which I doubt because they will probably say it was a holiday home given your use of it you will have to pay CGT on the gain. It may be that they will accept that the Dorset property was your main home from when your daughter moved close by, assuming you had no work tie to Surrey at that time.
If you did get the tax office to agree that the Dorset property was your main home from a certain point in time, that means that the Surrey property ceased to be your main home from that point and if you sold, that, part of the gain on that would be taxable. If you intend to stay in the Surrey property for the rest of your lives, the tax issue becomes academic.
Take a look here
for more information on CGT and the main home.
I hope this helps but let me now if you have any further questions.