Hello. I need some advice from anyone who knows much about domains and web hosting/buying url's.

Basically i have a main domain theplasticashtray.co.uk which i use for my portfolio (design). A friend set up a sub domain which i've been using to set up an online magazine blog.theplasticashtray.co.uk. Basically my web hosting has offered me these options for buying a new address http://www.theplasticashtray.co.uk/drop/options.jpg and im a bit confused as then i try the top option, it only points to my main domain rather than my sub domain.

Has anyone got any advice or know of any reputable cheap companies to sell addresses and can i buy one then just forward it to my .blog subdomain? I'm not that web savvy to be honest. Any help would be appreciated!