There are two legal jurisdictions in the Channel Islands, the Bailiwick of Guernsey and the Bailiwick of Jersey.
The law in Guernsey differs from the law in the UK partly because the Common Law developed differently (more Norman influence on Guernsey than on England) and partly because Guernsey and the UK have different legislatures and so pass different laws.
Until recently the UK was a member of an organisation based in Belgium and Luxembourg called the European Union which has its own legal system and courts and which requires member states to enact identical legislation in certain matters including, in particular, employment law. Although the UK is no longer a member of the EU most of its employment laws remain as they were in the EU-era. By contrast Guernsey has never been a member of the EU. So this is an additional reason for differences in laws.
Generally the parties to a contract can stipulate what law governs the contract and which country's courts have jurisdiction. For example a contract between a Guernsey company and an individual resident in England could be stipulated to be governed by the laws of France and subject to the jurisdiction of the courts of Barbados (this particular example is highly unlikely but I am just illustrating the principle). If the contract does not stipulate the law (or the contract is unwritten) then there are default rules which determine which jurisdiction's laws apply.
However, that said, most countries, including England and Wales, have some "mandatory" employment laws. These are laws which apply to everyone employed in England (or Wales) even if England/Wales does not generally cover the contract of employment. Mandatory laws typically cover such things as minimum pay rates, rest periods, minimum paid holiday entitlement, protection from unfair dismissal etc.
So some employment laws in England and Wales apply to everyone working in England and Wales whilst some other aspects of the employment relationship are governed by the law of the contract - which might be England and Wales or might be some other system of law.