The early XJ8s were designed to operate as you describe, only kicking the fans in with the AC which is fine when the cars warming up (in fact it helps the car warm up faster & use less fuel), however, Jaguar realised in some climates this was causing overheating, later ones (99my on) operate constantly with the ignition on at slow speed & speed up if it detects it needs to.

This is a complex 2 relay system with alot of wiring that goes into the ECU & cant easily be retrofitted to your one.

I`d advise you to take the car to a good auto electrician & ask him to fit a manual switch on the dash with a relay to the fans so you can turn it on manually when its warmed up (if he can fit a slow speed relay with a resistor as well as a full power one, all the better, as you dont want to over cool either).

I`d also advise you to get the thermostat changed every year & the water pump to be upgraded to the current spec one, as these are both known issues that occur & also cause overheating.

Best of luck!