4G on the iPhone 5 will only work on EE, so even if you got it from apple you'd only be limited to one network even after the other 4G networks start rolling out.
This is pretty much right, I would have added that Three/02/Voda will be rolling out a Another version of the iPhone5 which supports their 4G bands.
Apple may or may not produce another version of the iPhone 5 that works with whatever 4G bands these companies are able to get hold of. I find it unlikely though.
You will probably need a iPhone 5S or 6 to get 4G on any network other than EE in the future.
4G on the iPhone 5 will only work on EE, so even if you got it from apple you'd only be limited to one network even after the other 4G networks start rolling out.