Motoring (Official DVLA Site)
How to add higher categories to your driving licence (staging and upgrading)
If you hold a full car driving licence (category B), it may be possible for you to apply for provisional entitlements in a higher category. There are also circumstances that will upgrade a lower category on your licence if you pass the driving test in a higher category.
Staging
If you passed your driving test on or after 1 January 1997 and you want to learn to drive a larger vehicle, you may be able to apply for the higher category. Once you have obtained provisional entitlement on your driving licence you can take the test in that category. For example, a driver must hold a full car licence (category B) before being able to apply for a provisional licence to drive a lorry, minibus or bus.
This is known as staging and will apply from the smallest to the largest vehicles.
The full details can be found here:
Vehicles you can drive and how old you must be to drive them : Directgov - Motoring
Scroll down to the bottom of the page and you will find details of what you need to do to upgrade your licence BUT in summary:
Get yourself a provisional licence then go to a driving school for advice, pass your medical and eyesight test - (if you need one), pass the theory test, take some lessons and then take the driving test.
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Mo and Dick
Youth and Talent are no match for Age and Treachery
Remember: 'Life should NOT be a journey to the grave with the intention of arriving safely in an attractive and well preserved body, but rather to skid in sideways - screaming 'WOO HOO, What a Ride!'