I know you have a lot of time before you can do anything. However, having been in that situation can I share with you what has worked for me?
Find the two best Marriott locations for your trip. If you cannot book a reward stay at first, book a paid stay, (and be totally prepared to pay). Then begin your attempt to convert the paid trip into a reward trip! Things change all the time and I once had to check everyday for a month, it worked, and I got my stay on points.
If your go it will be fun, and rooms will be tough to get, so good luck!
There is no gainsaying the fact that, even now, hotel rooms in the centre of London are at a premium. You might be lucky, but don't bet on it! One option I floated in a post I submitted at the start of 2011 (I think) was to consider Marriotts within a 40 mile radius of London. The logic to this was/is:
1. Several events are sited outside of central London; e.g. rowing, tennis, equestrian. There are Marriotts nearer to these sites; e.g. Hanbury Manor, Twickenham, Maidstone Tudor.
2. The "outer ring" is more likely to still have some availability at this stage.