I'd be interested to know who was pro-active here -- did you ask about a paid upgrade or did they offer? (I've never asked or been offered.)
I called before I left home to ask if I had ben upgraded and was told I had been, from Superior room to Luxury room.
I asked if they had any suites available and was told the upgrade would be £40 from Luxury room to Junior Suite.
I then asked if I could pay the upgrade from Superior to Luxury (£25) in order to get the upgrade to a Junior Suite as the website was not showing any Luxury room when I booked, and they agreed.
Interesting (and several posters in the specific property thread have expressed the opposite view). One factor muddying things is definitely the (for outsiders) unpredictable pricing. I've paid from £69 to £200 for a standard room at the Sofitel -- and you can be sure that the times it was nearer £200 I checked the T4 Hilton prices first and found them to be just as high or higher.
For me the fact that the Sofitel had no pool was the main problem as I usually travel with my wife and child. I normally book the Hilton quite far in advance and pay £100-120 for a standard room and get an executive with lounge access. The lounge may not be as nice looking as the Sofitel but the snacks are much better.