Please define "non existent". Does the pedal sink to the floor, or pedal travel is normal but extra force is required?

If the pedal sinks to the floor, that would indicate something wrong in the hydraulics, which doesn't seem very likely as you say the brakes recover with higher engine speed.

I'd suspect a problem with the brake booster, as the power assist reduces pedal effort. On a gas engine, the boost muscle comes from intake manifold vacuum. The vacuum varies with engine RPM and load, so the booster has a built-in reservoir to provide assist when the vacuum supply is low.

You have a diesel, right? I've zero experience with the Jaguar version, but on other diesels on which I've worked, the brake booster got its muscle from either an auxiliary vacuum pump, or the booster was a completely different design that used hydraulic pressure from the power steering system. If the Jaguar version is either of those styles, the muscle force is RPM dependent. If so, you could have a problem with the muscle supply, or a problem with the internal reservoir designed to compensate for periods of low muscle supply.

It may just be a coincidence this happened after reconnecting the battery.