Yeah that's not that much, certainly not enough to warrant being called a "massively failing economy", otherwise so too was the US a massively failing economy in the 80s when Japan was buying everything up. Total property assets in the UK are worth £8.7 trillion. It says more about Qatar than it does about the UK, other than that the UK is a liberal capitalist democracy.
And it's quite the turnaround for Qatar in the last 50 years considering how they were once part of the British Empire. The Qataris still have a fascination with British things (e.g. Harrods).