UK mainland courier services do not include parcel delivery to places that are not on the main island, like the Isle of Man, the Channel Islands, the Isle of Wight, and the Isles of Scilly. This also includes Northern Ireland, and remote areas of Scotland.
Isle of Man postcodes are IM, Guernsey is GY, and Jersey is JE. Some PO postcodes are for the Isle of Wight, and some TR postcodes the Isles of Scilly. Northern Ireland has postcodes starting with BT.
The other area that most couriers exclude from "UK mainland"is the Scottish Highlands and Islands, which includes postcodes starting withIV, HS, and ZE, and some AB, KA, KW, PA, and PH postcodes. This is simply because these areas take more time to reach. Since they are also not so densely populated, parcel collections or parcel delivery is less frequent and therefore becomes a more expensive journey due to lack of volume.
BFPO (British Forces post office) addresses can only be reached by using royal mail services.