With you being in the UK, there is no online form to order backups from dell. With that in mind, your only option is to give dell support an ring. There is an support journal below,   [URL]    Or here for more contact details for dell uk.   [URL] 
Hi all, My Inspiron 9300 has been infected with a trojan horse and I need to reinstall Windows XP (Home SP2). When I sat down to do this I realised Dell never sent me the Windows OS disc, only the Dell Drivers and Utilities disc. As my warranty has expired, I am having trouble finding the correct place to go to reorder my disc - actually it's not a reorder since I never received it in the first place. Any help you can give me would be great as I have a £1600 system (4 years ago) lying useless. Thanks, Darroch Kaye
Reordering Windows OS Disc from Dell