Cambridge no brainer.

Amazon is not a particularly good company to work for, and if you could land an offer there, surely you could land a better offer by the time you graduate.

The opportunity cost here is very minimal if you think slightly longer term. In fact the doors that Cambridge might open down the road could far exceed whatever money you save as a L3/4 at Amazon.

If it was Cambridge vs a frontier lab or HFT offer of > 400-500k then it might be a harder decision, since it's likely that those are still the top jobs you would want after graduation.

Cambridge masters is much more of a once in a lifetime opportunity than Amazon is. People who tell you to go take the job probably are projecting and just want a job themselves.