It seems one thing after another keeps my plans on hold! I am currently a US resident, but will be moving back to the UK in February next year.

If I start something up over here, then how do I go about relocating? If its registered here already, then needs to be registered over there too.....

Are you talking about online business? I think registration would be required for tax purpose for specific country you're relocating.

That sounds correct - registration (for tax purposes) should be required for whichever country you're residing in.

Before you leave the US, check with a US business lawyer about maintaining registration in the US: see if it's necessary, see if it's possible to do and yet not pay taxes to both countries, see what you have to do to "close down" your US registration so that, if you move back, you can re-register without running into any penalties or problems. When you register in the UK, check about the same thing. You don't need to know all the details, just what you'll need to do regarding your business registration if you re-relocate back to the US for a long period of time.

I would think you would definitely have to close down your US business and then reopen for a UK business. Otherwise, you might end up paying taxes for both countries and that would be nothing but huge headaches!