It is a fair amount of money for an average family, but it's a perfectly reasonable price for a dress that the wife of the Prime Minister is going to wear on one of the most photographed days of her life.
The multi-millionaire thing is clearly bullshit, but the Leader of the Opposition earns £144,000 and the Prime Minister gets £160,000. They can afford a frock that costs a couple of hundred quid, especially if they're old enough that they bought in London decades ago so aren't burdened with a ruinous mortgage.