I received an email yesterday from American Express yesterday offering 5,000 MR bonus points if I spend ?2500 in the next 3 months. Here's the link they sent me, which requires logging into your MR account.
It appears to be targetted, so check your inbox. You might not receive one, you might receive a better offer than mine.
My expectation is that these points will post the instant you trigger the ?2500 spend, as recent AMEX promotions have done recently. So my suggestion is to use the 24 hour cooling off period on British Airways tickets like I described here to trigger your spend, then cancel the ticket. I'll be doing the same after I publish this post in the morning and I'll report back in the evening!