Why are Thatcher family allowed to flog off a government red box for £242k at auction?

Wondered why the Thatcher family were able recently to sell a government red box for £242k. Whilst the box was no doubt given to the former Prime Minister, it seems a little odd that her family are able to profit to such a degree from the sale of what was state property? I know Thatcher herself flogged off a lot of the nations assets, such as via the privatisation of water, gas, electiricity and telecommunications but surely this is a bit different?