Basically I want to create a fairly large terrain ( like the size of britain ) Anyway, what I was planning on doing was creating a big grid of heightmapped terrains. Of course I can't have them all displaying at once. So how would I go about making this work? I was planning on doing a visibility of 3x3 and just loading in new heightmaps/terrains as the player walked, but would the game spiked every time the new terrain is built?

Yes, there's always going to be a performance hit when you start loading, not to mention you have to predict when the best time to load a section of terrain into memory would be. One option to lessen the visible blow to the frame rate would be to place the loading in another thread so it loads in the background without the player even realizing it.

yeah i was thinking of doing that, i was also thinking of instead of deleting the old terrain and creating a new one, i would just translate the old ones into the correct position, and modify the height and color data, etc.

That's an option. Another idea I've had on this in the past is to create the world out of many height maps tiled together, and use a quad tree for visibility.

You'll also want to think of the possibility of the player gazing into the distance. In that case, there's way too much data to render with a reason frame rate. So you'll have to setup LOD, and fix each height map to tile seamlessly with its neighbor even if they have differing LOD. Things in the far distance, though, can simply be rendered to a texture and displayed way out there.