Hi MyRideGear,
The only place within your Auctiva account where you can change your time zone preferences is by clicking the "Account Settings" link under the "My Account" tab within your account. Therefore, if you want to change the time zone reflected in your account between scheduling, for example your eBay US and eBay UK listings, you would have to do it from there.
Since there are only 3 time zones that are relevant to you, it maty help to create a reference chart in Excel or a similar application that displays the time in AEST and BST for any given PST hour.
-Mike