Was that Daisy Ridley's body

It was Daisy. There was no body double used. Body doubles are rarely used for guest starring roles where the actor in question is not a well known star. The casting call for the role almost definitely stated that partial nudity would be required. If Daisy had been unwilling to do that, she wouldn't have been cast.

A lot of people have questioned whether or not it was actually Daisy in the scene because the way it was filmed. This was likely due to the nudity rider she had agreed to. Nudity riders are very specific. Daisy's likely said that any time her face was clearly visible from the front, the shot would be framed in a way where the nudity was only implied. In all the shots of Daisy's body, she is either filmed from behind at an angle, or from the front where her face is out of frame.

Having said all that, the nude scene in Silent Witness is about as tame as they come. It's non sexual and not at all gratuitous.