Hi, I assume you're a British subject. So your embassy will issue for you such a document. If you're an American, forget it.
I'm Canadian, and had the misfortune to need such a document many times. It is just a form letter run off a computer which salutes the other embassy where you're requesting a visa, and states that you are indeed the person shown in your passport. Each time the letter, run off by a lowly employee, cost me Can$50.00 (Us$35.00). Your embassy does no cross checking. All they want is your passport and your money. So fear not. Your unpaid parking tickets will not be used against you.