A Guernsey Lawyer will be more aware of the contract you were employed under as your agent did not work it out themselves they have used legal representatives to design your contract and it terms.
A lawyer wrote your contract who is aware of contract law and it terms.
Not so many lawyers dealing with Guernsey law in UK as its different from our laws there is special terms in labour contracts not only taxes which is why offshore workers and seaman and other worldwide workers from UK get contracts from there.
A UK law firm with offices in Guernsey would be also fine to use but I would not use a lawyer in UK without an actual office there.
As the work is going to have to be done there in Guernsey.
