Can only speak from an Ambassador guest point of view but I have stayed several times and always had very good upgrades. They do from what I tell prioratise upgrades to Ambassador guests and then Titaniums and they do get quite a high number of status guests for the size of the hotel which is small. Overall however I feel if they have a better room / suite available for your stay they will give it to you and wont try and avoid upgrading elites.

I have had several really good stays this year and its become my first choice for when in London!