In terms of redundancies, it may be best to go with voluntary first to see if you will get some who will take it and advise staff that if you don't get the numbers you need you will then move to compulsary. The plus side of that is that if you get the numbers voluntarily, you avoid all the issues of a compulsary redundancy situation. The downside is that you may have to pay more then statutory to make it attractive, whatever you offer under the terms of the voluntary scheme needs to be offered to staff being made compulsory redundant and the people who may volunteer to go are not always the ones you want to leave
If you move to compulsory redundancies, yes you can identify people but the reasons you use for picking those people have to be fair and seen to be fair. Certain reasons (eg someone on maternity leave) may be illegal.