Travel loyalty is what I do for a living. 🙂 However, even I make exceptions for London (and I've traveled there at least five times a year over the past decade). That is where I spend my Chase Ultimate Rewards points.
Again, it makes me very happy that points is driving your purchase decision. That's why we give them away. Just as insider advice, I'm letting you know that Central London is a unique market for Marriott and Hilton. It has a very high influx of international business travelers, who disproportionately fill their properties precisely because of their Loyalty programs AND has many high points earners seeking to redeem in the very same properties. It really drives up prices for the remaining rooms for the rest of us (leisure travelers, moderate point earners, business travelers on a limited budget).
If I had $1,300 for four nights in London, I'd either stay in an incredible apartment (booked through Chase) or else at a Montcalm or Montague Hotel.
If you're over the weekend though and want the most value for money from Marriott, I'd look at their Canary Wharf property.