You have to be careful with UK Police Clearances, they can come back with no trace or no live trace. The UK has a system where old offences are not listed on your certificate, however it will say no live trace which implies that there was an old conviction.
mine just says no trace, but in the forms i had to fill in on the visa application it said have i ever been arrested and i answered yes, even though there was nothing on the certificate, as i know that if i had said no and they find out i had been then thats misrepresentation and i could be banned from canada. so i guess they just wanted to know what the arrest was for and in what country, but iv sent the letter off and all should be sweet hopefullllllyyy