1) Impossible to tell.
2) At the end of the day reputation doesn't matter, what matters is what you do with your education. You can have all the PhDs in the world but if you can't make money.... My present concern with Oxford would be their connection with the Man Group which isn't doing so well at the moment. Not sure if that will have any impact now or later. Regarding Harvard, the question really is US vs UK? PhD from Harvard would be worth more in the US than Oxford/LSE (my guess only).
3) Once you're in the job, look around at the credentials of the people working there and if you really trust someone there, ask their opinion (I would be vague on time frame but ask if they think having a PhD is worthwhile in the long term).