Will felony convictions in USA 11 years ago affect a UK Citizen applying for a job in the UK now?

If a UK Citizen has felony convictions in the USA that are 11 years old and is applying for specific types of jobs in the UK now such as a Firefighter or an EMT should they divulge this information when applying and if so can/will it negatively affect their chances at being hired? Can a person in this situation even become a Firefighter or EMT?