You can list on the .co.uk ebay from the US.
Here are a few options:
1) just add your listing making sure you choose that you want to sell to the UK. USA results show in the UK results over here.
2) Just select the UK ebay version. It is only using IP detection and they always allow a way to look at either version of the site in case the IP detection does not work. Don't know where the link is on the USA version but it is there.
3) Use a proxy to access the ebay uk website
4) Change your browser settings (firefox have plugins) to look like you are in the UK.
Now, if you are saying that you want to sign up / run a UK account that you don't currently have, that is a different story, but to be honest, you should be fine with option number 1 above.
Hope that helps.