If they are coming to France and intend to remain employed by a UK company, their employers need to be very careful as the French tax authorities might assume that the UK companies for which they work will have a theoretical office in France from which each will be employed, with French tax and social charges applicable (unless they are “posted” to France on secondment, which may not be possible from the UK after the transition period).
However, this is a very complicated area and professional advice would be appropriate.
As to any self-employment options, it depends on the kind of work and the level of income and expenses. There are three possibilities:
Micro-social with versement fiscal libératoire (auto-entrepreneur): This simple business set-up, not available for all activities, involves social charges and tax levied monthly or three-monthly at fixed rates on turnover.
In theory, the rates are beneficial, but it may not be the best option if expenses are high. Then there is the ordinary micro system involving tax on a percentage of the income after a set allowance for expenses, which varies depending on business category.
Finally, there is standard accounting, with income tax at the banded rates applied after deduction of “real” expenses, and social charges taken at a standard amount of around 34% in all.
Speaking to a professional and providing specific details will help ensure the right choice.