I can't post a link to release package 280, but release 224 seems to also contain the theme. Therefore it is no problem to install the theme. Goto http://updateservice.t-mobile.net
, select T-Mobile UK and login with the following credentials:
Select Home > Handheld > 8700 > Handheld Software 220.127.116.11. Download the firmware package for the 8700g and install it on to your pc. Copy the file net_rim_theme_bb_dimension_zen_320x240.cod from C:\Program Files\Common Files\Research In Motion\Shared\Loader Files\8700-v18.104.22.1686_P22.214.171.124\Java
to a temporary directory and upload it to your BB using javaloader.exe.