I'm not sure what currently applies to the London embassy (anyone?) however, when I went through London last year I didn't attach any photos to the forms. They did that at the embassy during the interview. The forms were sent back photo-less when returning P3.
I do know that it has been done both ways (some attach photos and some don't) and there has been no issue. Just remember to bring the photos with you if you don't attach them yourself.
I'd be a bit wary of sending them in with the forms unattached. These things can easily get lost and if you got to the interview assuming you'd sent in the photos and they'd lost them AND you had no replacements...that might be a buggar.