Of course, they will most likely all be co-authored pieces, but the significant point is that the REF rules, except in special cases, impose no penalty on genuinely co-authored work; they explicitly state that it is welcomed. In most cases, there is no disadvantage in submitting a co-authored item to the exercise (although there is some complication when co-authors submit in the same return); it is not as if it counts as half an output or less.
The REF does not discount coauthorship