There is no major disadvantage to admitting the adultery. The UK divorce system is based on no fault - so the admission will not harm you in the financial settlement.
If you do not wish to admit it then your wife will have to prove the adultery (which is actually very hard - unless she has a video tape!).
More likely she will the re-petition with another reason - more than likely unreasonable behaviour.
Re: your girlfriend.
Is she actually named as a co-respondent?, this is unusual these days as it is against standard protocol.
She will not have to go to court unless she is named as a co-respondent and your wife can give a very compelling reason why her evidence / attendance at a hearing is necessary - this is very very unlikely.
(please read disclaimer re: any advice given on these forums)