My idea has recently been granted a patent in the US. I am based in the UK. Can I still develop and market the product in my local territory? I assume the US patent holders could apply and be granted a patent in the UK and then insist I stop selling the product (or offer a license agreement). But until then am I free to continue with the development?
If there are no other patents, you can do anything you want with the product as long as you don't produce in or export to the US.
There might be laws against unfair competition in the UK preventing some types of copying, but patent law is territorial to where the patent is valid.
The published US patent/application is prior art to any UK applications that are filed now, so if they haven't yet applied for a UK or EP patent, for example by claiming priority to the US patent, they can't do that anymore.