Given that the first RAF Chinooks date back to c. 1980 (pre Falklands War) and that they are now 30 plus years old it is hardly surprising that we have decided to buy some replacements.
Madbob,
You are correct, but the original ZA and ZD s went through a mid life update to D model standard in 1993-94 so they're not exactly in original condition. Still a great aircraft!