Cambridge.

You'll have no problem getting the Amazon job (or similar) with a masters degree from Cambridge afterwards.

I doubt you'll retire wishing you'd worked just one more year at Amazon.

(I have a masters from Imperial. An equally good alternative if you'd prefer to live in London, or prefer somewhere a little more business focused. Google and Amazon both skipped the first interview stage based on my degree.)

Also if you ask Amazon, they're very likely to keep the job offer open for a year.