Quick tax question

I know this may have been answered in another post, but I couldn't find it. Sorry if it is repetitive.

This was this first year that I've needed to file and I've prepared the 1040 and 2555 forms. Since I worked only in the UK for a UK employer, I don't have a W-2. Should I wait until I get my P60 to send in with the forms? Alternatively, what proof would I send of my earnings?

I don't include any proof of my UK earnings. I just put the amount on the form. The P60 is useless because it's based on a different tax year.

Thanks!