Great post Orphalesion! I especially agree with the Roserade idea.
Below are some Pokemon from previous generations that I feel fit into the Great Britain theme and could possibly work as regional variants.
Galarian Lapras
Lapras had an original proposed English name of Ness, named after the Loch Ness Monster. So likely was themed after Nessie. Would be a great Pokemon to do a UK variant.
Galarian Granbull
Based on a Bulldog, very British. Could see this either being brought back into this gen or given a regional variant.
Galarian Swanna
Swan's have a long established connection to British royalty. There's this whole "Swan Upping" thing, sounds like they count and examine all swans along a 79 mile stretch of the Thames. Either brought in as is, or regional variant would fit the setting.
Galarian Rapidash
Sword and Shield is looking to possibly have some knight related themes. A regional variant of Rapidash would be amazing, maybe becomes a warhorse with armour (Fire/Steel).
Galarian Pyroar
England and Scotland both use lion symbols. Would suit a UK variant.
Galarian Aegislash
Gen 8 is literally called Sword and Shield. We have a Sword and Shield Pokemon, so could maybe see a regional variant of Aegislash.