It's up to the courts to prove that you are guilty of "driving without due care and attention", or guilty of "dangerous driving".
It is not illegal to drive in big boots, or as Santa Claus, or even inside a sleeping bag. It is illegal if they can prove that it is dangerous, or without due care. If you can prove that you can drive safely (access all the controls and pedals) you will win and they will lose. It is the same laws that apply to having a sat nav in your field of vision, or having an argument with you wife, or adjusting your radio.
Despite the onus being on them to prove guilt, a "right thinking" person would probably consider that you were not able to drive safely - so you would have to prove that you could.
Pistonheads SP&L is a very good place to go if you want to have proper answers from actual lawyers and barristers.