As Boxtops indicates, the UK Border Agency has a computer system which flags up anyone who is wanted by the police, resulting upon their immediate arrest upon trying to enter the country.
For minor assaults, falling under the category of 'common assault', the law states that 'the information must be laid' before a court within 6 months of the offence. More serious assaults, such as ABH or GBH, are not subject to such a restriction. There is no law imposing a time limit (of 2 years, or of anything else) upon when an arrest and charges can result from such an offence.
Chris