The Building Itself Probably Only Has Scrap Value.

From The Daily Mail:
A mansion on London's Billionaire's Row which was built by the owner of the Tate and Lyle sugar empire has sold for £25million - just a quarter of its original asking price.
Heath Hall on The Bishops Avenue in Hampstead, north London, a 24,000 sq/ft trophy home set in a plot of around 2.5-acres, was put on the market three years ago for £100 million.
If you look on Google maps, the plot appears to be about 100 yds x 120 yds, is nearly rectangular, with a road along the back for the houses behind.
So you could build a row of about fifteen nice big terraced houses along the front (on The Bishops Avenue itself) and another ten along the back, with off street parking for two or three cars in front and decent sized back gardens (by London standards).
You can sell each finished house for (say) £1.5 million (Zoopla's Z-Index for that postcode is just under £1 million), knock off a couple of hundred thousand for the cost of building it, £1 million for the land (£25 million ÷ 25), the rest is your profit margin, Bob's your uncle.