Ok, I just gave some random numbers but even if it is less than I originally put, I imagine, that they would sell much much more of those phones, if those were available on other, major networks and unlocked.
Without knowing the exact details of the exclusivity deal there is no way of us knowing if it was better for Nokia to sign exclusivity in the UK or not. It's not just the money EE paid for exclusivity, it's the marketting and the carrier commitment too.
The Lumia 920 is a top end phone and having it on the first LTE network in the UK would be deemed as a necessity due to the hype EE's launch has been getting. If EE demanded exclusivity or it wouldn't have the device at all then Nokia were stuck between a rock and a hard place. If they refused exclusivity then it would lose out on the 4G market big time to the GS3 and the iPhone 5 and that would hurt them badly.
If EE promised better marketting support and commitment to the Lumia brand then competitor networks as long as it launched exclusively then Nokia would lose out in terms of the public's mindshare if they didn't go for the exclusivity deal. EE have been enthusiastic about the Lumia in their PR and marketting building up to launch for sure.
On the other side of things, Vodafone doesn't seem to have committed to any WP8 phones yet, so chances are they didn't want it. It seems O2 only has the HTC 8X, not even the 8S according to their website, so they may well have opted to only go for a single WP8 device to test the waters and the 8X is the cheapest top-end device. Three representatives seem very enthusiastic about the 8X too, but seem to be recommending buying it sim-free and getting a sim-only contract through them.
If this is all the case and the other major carriers were less than enthusiastic about the Lumia 920 then the exclusivity deal with EE was a very good business decision for them as they get EE's full uncompromising support and it would work on T-Mobile & Orange which are very strong brands in their own right. It makes it very frustrating for customers of other networks wanting to buy it sim free for sure, but it could well have been the best option on the table for Nokia as they're at least guarenteed X number of units while relying on sim-free sales would not guarentee them anything at all and would be a much bigger risk.
Everyone likes to report on how badly Nokia are doing as a business at the moment, so it makes sense they would take the safe option which is guarenteed a minimum return against the risky option which may get them more sales or may end up tanking and hurting their stock prices even more.
I'm not defending Nokia by any means, I would prefer it to be available on all networks so the choice was there, but if you're to talk about it in a business sense, debating how much they could sell with or without the exclusivity, you need to look at it like a business decision with all the various factors involved and not from a consumer's point of view. It's almost certain that what I've mentioned here were just a small fraction of the issues Nokia took into account when making the decision to be exclusive to EE. The fact they've only got exclusivity until the new year with an option to extend it shows that Nokia themselves wanted a get out clause in case the deal wasn't what they were hoping for. If EE doesn't meet their expectations then they almost certainly won't extend the exclusivity deal.