...Only EE and Three have any 1800MHz at the moment, and if there is any more free (not sure on that) OFCOM aren't planning to auction it till next year. By the time it rolls out I'd imagine the 5S will be available.
I believe that O2 and Vodafone do own some of the 1800MHz spectrum, but the amount is so tiny that they cant move existing 2G/3G services already using that band to other ones, in order to free up enough space for 4G services on 1800MHz.
The next 4G auctions will be 800MHz and 2.6GHz spectrums, which the current iPhone 5 doesnt work with as you say, the 5S might be out, but its also been suggested (this was analysts, for what thats worth) that Apple will bring out different models to work with those spectrums. Either way, it looks like some might be locked in.