Julie Andrews' Former London House On the Market

I've always thought the convention was you needed at least 8,000 square feet to be called a mansion, T&C should know better but I suppose if you are asking £24 million you'll push the nomenclature envelope.
From Town & Country, Apr. 24

Julie Andrews's Former London Mansion Is On the Market For Over $33 Million

The Mary Poppins star lived in the stunning home back in the 1970s.

The breathtaking mansion in Belgravia, London that once belonged to Julie Andrews and her filmmaker husband Blake Edwards is now on sale for a cool £24 million, which is over $33.5 million in USD.

The magnificent Grade II listed property has had a multi-million makeover since the iconic Mary Poppins star resided there with husband Edwards and their children in the 1970s, and is now on the market via luxury estate agent Rokstone.

Built in 1835-36, under the auspices of the Duke of Westminster, the newly refurbished six-bedroom townhouse mansion – and linked mews property in Chester Square – is one of the most luxurious in south-west London's Belgravia. Covering 7,005 sq ft, the historic house boasts vast reception rooms, a private cinema, a gymnasium and a private spa to wind down in afterwards, including a jacuzzi, steam room and sauna....MUCH MORE

Here's the listing at Rokstone who don't use the "mansion" terminology, going with "Belgravia town house" instead.