See Main Topics as appropriate, for example:
EUROPEAN LAW/directives etc/freedom of movement of workers ;
European Law/judgment enforcement (this note also covers jurisdiction of EC Member States)
OVERSEAS NATIONALS (employment of) .
UNFAIR DISMISSAL/employments not having unfair dismissal rights/work ordinarily outside Gt Britain
The question of the extent to which employment tribunals have jurisdiction to consider cases in which an employee works abroad and/or is employed under a contract governed by non-British law is currently (September 2003) in a confused state - see notes at Overseas employment/unfair dismissal etc - new rules .
NATLEX, the ILO database of national labour related legislation is a useful resource for international "labour, social security and related human rights legislation", albeit mainly concentrating on social security aspects.