I think they deliberately chose not to use British accents because they wanted the characters to sound relatable to modern audiences. for example, they wanted Hercules to seem like the kind of guy you could have a beer with and talk about regular boring real life struggles, rather than some larger than life action figure who terrifies everyone around him. Similarly with Zeus in the original Hercules telemovies - Anthony Quinn didn't play him as a big booming shakespearean character like Laurence Olivier did in Clash of the Titans, he played Zeus as a regular old guy who stops and chats to villagers in the street and wants to have a picnic with his grand kids. They were turning the Greek myths on their head, and making the shows feel like a family drama that just happened to be set in a fantasy world, rather than a fantasy series with bits of family drama in it.

Whereas with the Lord of the Rings movies, they did take the fantasy aspect a lot more seriously and went for posh Britsh accents, particularly with the elves, because they didn't want Arwen to seem like some chick who might have been working in a record store, she's a princess from a mystical woodland. We're not supposed to relate to her or Galadriel or Elrond, we're meant to be in awe of these supernatural beings. The Hobbits talk like regular farmers from the British countryside, because that's the sort of environment Tolkien would have been living in, and we're seeing this world full of big, exotic, mysterious people and creatures through their eyes.

The Xenaverse shows were aimed at 90s audiences who were used to watching American shows and movies, and just wanted something a bit more escapey.