When the British make a plan, they are setting a direction, not writing a script. The plan establishes where they want to get to and identifies the major steps along the way, but the expectation is that the route will be adjusted as they go. A plan that never changes is not a sign of good planning—it probably means the plan was not responding to what was actually happening.
The person executing the plan is expected to use their judgment about when to stick with it and when to modify it. If circumstances change and the plan no longer makes sense, you change the plan. Stubbornly following an outdated plan is not considered disciplined—it is considered inflexible.
The plan serves the goal. When the plan stops serving the goal, the plan changes.