Adultery, in respect of a UK divorce, is an incredibly detail-specific topic.
The exact definition refers specifically to penetrative sexual intercourse between two people of the opposite sex. Bizarrely, if you have homosexual/lesbian penetrative sex, or commit other hetrosexual sexual acts, one can only proceed with divorce on grounds of unreasonable behaviour.
You do not need to cite the name of the other person involved but you must go some way to prove adulterous acts took place.
Interestingly enough, adultery can take place long after a couple have split up, showing that not to be the cause of the marriage breakdown in the first place. The law states that the Petitioners inability to live with their marriage partner, need not directly relate to them eventually having had an adulterous relationship.
Ultimately, whatever acts have occurred with whichever party, the divorce and financial settlements are likely to remain unaffected by adultery. It will not tip in favour of the affected party and if the accusation is contested in a counter petition, the only person celebrating will be the solicitor. If things have broken down to such a degree that mudslinging is commonplace, take it on the chin and just get it over and done with; divorce is no fun however you cut it.