Special cases: Class 1 - mariners: DL contracts
The place where the contract is entered into is the place where the offer is accepted.
Postal acceptance is a basic English contract law principle. If the contract document is signed and posted back to the employer in the UK acceptance is complete as soon as the signed document is put in the post box (a binding contract is made even if the document is lost in the post).
If the signed document is posted from overseas the postal rule may only apply when received by the UK postal system. The contract then becomes entered into in the UK.