Are you completing an application as well? In the Uk that's the part that's important. Some places allow you to submit a cover letter and cv as well as an application, but almost all of them require an application where you state how you fit the person spec.
I have the option of filling out the online application or uploading a single file containing a letter, CV, and research statement. I'm opting for the latter, and using the letter to explain how I fit the person specification.