Hi Adam,
As long as you list your items to the eBay UK site, the prices will be displayed in your Auctiva store in pounds sterling.
However, there is no way to make your listings displayed in your Auctiva store to reflect GMT. If a customer clicks on the eBay listing, though, he/she'll be directed to the actual auction, which will have the end time listed in GMT.
There is not a way to make the link on the store window direct user's to your eBay store.
I hope I've covered all the bases=)
-Mike