Yeah any fireplace connected to the floo network is accessible via floo - so you can go from one end of the UK to the other.

Apparition depends on the skill level of the witch or wizard, and also how familiar they are with the places involved. Trying to apparate into a bedroom they know every inch of is easier than apparating into a stranger's house safely. It's not that it wouldn't work necessarily but there's increased risk of splinching or other mishaps if you're pushing the limits. One of my characters got side-along apparated a big distance once and she was livid because apparating that far felt unsafe to her. Ben Crouch won't apparate if he can avoid it and never if he's had anything to drink because his brother got splinched apparting under stress.

Some places also have doors that open into other countries though, eg Atlantis has a door in France and India so people could travel that way.