UK cold emailing - cover letter attachment or main body text

1st Yr Undergrad at a UK target - i have secured 3 spring weeks however none are at financial institutions (main one is at an energy trading firm)

I am eager to secure something more substantial (and at a more reputable place) over the summer and so I am cold emailing certain firms (with alumni presence)

Do I include a Cover Letter as the main body of text or should I include the Cover Letter as an attachment (letter is about 400 words)

thanks :)