There has been some interesting discussion about why the computer video evidence of the rape has been introduced here when in the Vincent Tabak case, his viewing of violent porn was hidden from the jury. This is called bad character evidence and as has been mentioned, the prejudicial effect should not outweigh the probative value. The law on this comes from the Criminal Justice Act 2003 and basically, legal argument is presented to the judge before the trial on this, as well as other issues, such as hearsay. Bad character evidence of a defendant can be introduced in 7 different ways. One is if all parties agree it should be introduced. The second is if the defendant provides the information (perhaps to show s/he is a reformed character and is being honest). Another way is if the defendant attacks another person's character.
Now what I am going to say is my opinion only. The judge in the VT case ruled the evidence to be too prejudicial, possibly because watching videos of women being strangled does not show you are in the habit of killing women yourself. However, I became very aware when reading the court transcripts that his barrister was exceptionally careful to avoid saying anything negative about Joanna, even though there had been no witnesses and VT could have blamed everything on her. This led me to suspect he had something to hide that he didn't want to come out through attacking her character. I guessed it wouldn't have been killing someone as that would probably have been introduced into court but it must have been something that could have been damaging to his case.
With regards to NM and SH, attacking a person's character not only in court but also on being questioned under caution or after being charged opens the gateway to bad character evidence being introduced, so I would guess this is why the information about the rape video has been allowed in court, especially as there are substantial similarities between the video and what allegedly happened to Becky.
All my opinion of course. Sorry for such a lengthy post!