According to The Big Issue (I can’t find where they got their numbers from) there are/were more than 20 million square feet of empty office spaces in London. Which if we extrapolate from data I can find on occupancy rates within just the city of London, equates to around 5% of the total office space.

Not really a particularly large amount when you also see that roughly 7-8% of residential properties are also currently unoccupied.