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:
Username: tmobileuk
Password: tmukbbcorp
Select Home > Handheld > 8700 > Handheld Software 4.1.0.224. 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-v4.1.0.266_P2.0.0.111\Java to a temporary directory and upload it to your BB using javaloader.exe.