As far as I understood it...
You could have white crowns or fillings if they would be visible. However, from what you have said it sound like the only reason they need replacing is because of discolouration. In which case that would be having a procedure done for purely cosmetic reasons - which isn't covered.
If you want, or your dentist suggests, treatment that is purely cosmetic (for example, tooth whitening, large white fillings or white crowns on back teeth), you can decide to have this treatment done privately.
Taking from http://www.nhs.uk/AboutNHSservices/dentists/Pages/DentistsFAQs.aspx#q10 (http://www.nhs.uk/AboutNHSservices/dentists/Pages/DentistsFAQs.aspx#q10)
As I said that is my interpretation of the rules which seem to be looked at in different ways by different practices.
Privately: £760 for two crowns sounds about right by the way.
Sorry, I can't be of more help
Jon