An exit from the EU would lead to considerable uncertainty from an employment law perspective because key areas of employment law are derived from EU legislation and so might fall away automatically, be abolished or be amended.
These areas include working time, agency workers, fixed-term employee and part-time worker protection, health and safety, acquired rights under TUPE (Transfer of Undertakings (Protection of Employment) Regulations 2006), collective redundancy consultation, works councils, discrimination and certain family-related rights. Perhaps the primary targets for abolition or amendment (assuming that is achievable in the context of whatever new relationship with the EU is negotiated) would be the Working Time and Agency Workers Regulations, which have been heavily criticised as imposing unnecessary burdens on business. Of course some of these rights will have been transposed into employment contracts and policies and therefore would continue to apply until varied.
Where legislation is retained, a key question will be whether courts would continue to follow decisions based on rulings of the CJEU in those areas or be willing to re-examine and potentially overturn established doctrines. Much-litigated issues such as holiday pay could be re-opened, making the legal position unpredictable until suitable cases are decided by the UK Courts.
Finally, greater restrictions on the freedom of movement of workers, if negotiated, would work both ways. Seconding staff to work in Europe would be subject to more restrictions, while staff seconded to the UK from EU countries could also face more controls.