I can't see what everyone is worried about...all of the ZZxxx serials have been out of sequence (e.g. C-17s ZZ17x, just so '17' could appear in the serial), 51 Sqn RIVET JOINTs so they could get their 6+6+4+6+6+5+6+6+6=51 'signature' allocation, and so on.
The Typhoon fleet is somewhat nearer to reality (ZK3xx range), therefore fear not pretty much forever more with hardly any (if at all) serial blocks of ZL/ZM(except the F-35s)/ZN/ZP/ZR(a few helos)/ZS/ZT (one Lynx)/ZU/ZV/ZW/ZX/ZY being used so far.