Does any one know what will be regarded as an acceptable valuation of a vacant building plot in the UK which was still owned on taking up permanent residence?

About 6 years ago I bought a large property which included a small stable. Over time did the house up, demolished the stable and sub divided the ground. Engaged estate agents who valued the house and land and put them up for sale separately several months before we moved to Oz. Sold the house prior to arriving in Oz but still had the plot for sale. Finally took a loss on the sale of the plot to get rid of it.

I guess my question is will the tax office accept the valuation that I had it on the market with the estate agents as the value on the day of my arrival? If so then I will have a nice CGT loss sitting for use against the sale of other UK assets in the future.