No Starter Pokemon (except the Bulbasaur and Squirtle lines), Fossils or Legendaries/Mythicals/Ultra Beasts from past gens
You can buff said pokemon you're bring back if you want to like giving it a new ability and move they would've benefit from
I would add (and buff) the following...
They're like because
A) They fit the region and make sense
B) They're a counterpart of a already confirmed pokemon
C) They're a fan-favorite
D) The species said pokemon is based on is native to europe, let alone, UK