Jersey is part of the channel islands and not classed as uk mainland
to quote wikipedia, lol
"Since 1969, Jersey and Guernsey have operated postal administrations independently of the UK's Royal Mail, with their own postage stamps, which can only be used for postage in their respective Bailiwicks. UK stamps are no longer valid, but mail to the islands, and to the Isle of Man, is still treated as UK inland. However, it was not until the early 1990s that the islands joined the UK's postcode system, Jersey postcodes using the initials JE and Guernsey using GY."
hope this helps