There are a number of different work permits that can be applied for if you are considering sending US personnel to the UK.
Each EU country has it’s only rules in this area. EU citizens are free to work anywhere in the EU, post Brexit such rules are subject to change but realistically may not vary greatly in the future.
For US personnel coming to the UK, certain aspects should be considered. If someone is born in the EU but has never lived in the EU they can apply for a passport in the country of their birth, from this it’s quite straightforward to obtain a social security number. If someone’s parents or grandparents are born in the UK then ancestral visas are a possibility.
For other types of visa, emphasis should be made on the technical qualifications of the proposed individual(s). You would not see this officially from a Government agency but preferential treatment or fast tracking is given to people with specific technical expertise