DFC
I am afraid that we are short of codes, most of the European core states are (not Spain, Germany or France though - yet). The problem is that only certain codes can be used in certain areas. For example, domestic codes should only be used within the country that they are allocated to and then only to within 30Nm of the FIR boundary (PANS ATM or EUR CAL). Transit codes are used for crossing PA boundaries and then there are superdomestic and supertransit, sub domestic and military not forgetting NATO.
I am reliably informed that the UK may well have a problem prior to Mode S circa 2012 (ish). Hey, just in time for the Olympics! I actually believe the current plan is to replace Swanwick systems in 2012 too, now that will be fun!!!