Here's your letter. (Change the address, dates, etc. to meet your circumstances):
"To: The Clerk of the Court,
Blogsville Magistrates' Court,
High Street,
Blogsville,
Bogshire
10 March 2008
Sir,
I hereby enter a GUILTY plea in respect of the charge against me under Section 143 of the Road Traffic Act 1988, which is due to be heard before the Court on 15 April 2008.
I respectfully request that the Court is made aware of the following circumstances relating to this offence:
On Saturday 16 February I travelled to Mucktown to complete the purchase of a Ford Mondeo, registration number P123 ABC, having earlier made a successful bid for it on an internet auction site.
I realised that I should ensure that I had third party insurance cover before driving the vehicle to my home address but, because it was late on a Saturday evening, I was unable to contact my insurance company by phone. I therefore, unwisely, decided to drive the vehicle home without obtaining insurance cover. My intention was that the car should then remain stationary at my home address until such time as I was able to obtain insurance.
I fully accept that my actions were illegal but I wish the Court to be aware that it was my intention to obtain insurance as soon as I could contact my insurance company.
Yours respectfully,
Edward Dusius"