Welsh and Gaelic are not English dialects, they are distinct languages.
:: beating head against wall ::
It therefore follows that dialects OF ENGLISH spoken in regions with a strong underlay of a different subfamily will differ significantly from dialects in regions with a longer English-speaking history.
Ferries are more commonly used for transport than submarines.
"Surface transport" means "non-air". Please tell me you already knew this. You can put your car on a ferry. But you don't generally drive from London to Dublin. (London to Paris, possibly.)
The main UK political parties have no parliamentary representation in Northern Ireland
Oh, cool, so that's another area where the GB:UK distinction is significant. If your website is about British electoral politics, you'll want to know if your user is GB or elsewhere. But you'll also need to know if they're UK at all. A site based in the US might conceivably need to know whether you live in a state or a non-state (= territories plus DC) but in general, on average, the important distinction is either "continental US" vs everywhere else, or "lower 48" vs. everywhere else, depending on whether you charge extra for shipping to Alaska.