If you gash your planning by using 'still air' timing and IAS = TAS, you will introduce an avoidable error of at least 10-12% in even a 10kt W/V at 2000ft. This will mean that on a typical 50 mile leg you will begin the leg with an error already greater than that allowed in the PPL Skill Test.
If your TLAR/still air 'planning' results in an estimate greater than the maximum allowable for the PPL Skill Test before you take-off, you will fail the test before even getting airborne.
Your choice to be gash - but it takes a very short time indeed to plan using a navigation computer. Which is more essential the slower the aicraft, due to the greater effect of wind.
All PPL examiners have a mandate to reduce airspace violations by private pilots. To do this, we insist on the applicant planning correctly. I understand that the RN and Army do still require the use of a navigation computer for pre-flight planning, but for whatever reason the RAF does not?