You'll have no problem purchasing your ticket and leaving the US. However, you may have trouble upon trying to pass through immigration in the UK. It has a concept of "spent" convictions. So if an immigration officer wants to exclude a convicted criminal, they must prove that the offence is not spent and therefore the person is not rehabilitated.A conviction is deemed "spent" if more than 10 years has passed since imprisonment, if any, of between 6 months and 30 months. Any imprisonment over 30 months cannot ever be spent so it will always count against you. Imprisonment of less than 6 months and/or fines have an even shorter rehabilitation period  of around 5 years (or less).The immigration officer wishing to refuse you entry on the basis of a criminal conviction must prove that your conviction has not been "spent". The burden of proof is on immigration, not you. If the offence is spent, then you don't even have to tell them about it and they cannot use it against you even if they do know.