However, there are some circumstances where convictions are never regarded as spent. In particular, convictions are never spent when applying for jobs within, or alongside, the 'caring professions' (medicine, teaching, etc) or in the 'administration of justice' (police, prison service, etc). Also convictions are never regarded as spent in situations covered by the laws of other countries (unless the relevant country has similar 'rehabilitation' legislation). i.e. your conviction will never be regarded as spent when applying for entry to any country which requires UK citizens to obtain a visa.
Officially, you must seek a visa to enter the USA. However, most people simply 'forget' their convictions and enter the USA (illegally) under the Visa Waiver Program. The US authorities have no direct access to UK criminal records. They won't know about your conviction and they won't have your fingerprints on file. The only information which the UK is allowed to pass on, under EU law, is specific information about particular people where there is 'justifiable cause'. (i.e. the US authorities will know about drug-traffickers, child-sex tourists, suspected terrorists, etc. They won't know about you).
Chris