Whenever I want quick wisdom on a piece of Church of England jargon I normally turn to this book - ABCD (A Basic Church Dictionary) - compiled by Tony Meakin.
Under confirmation he says this:
'(Latin confirmare - to make firm). Confirmation is the rite by which a person, having been baptised and made the baptismal vows, receives, through the laying on of the bishop's hands, the particular grace of the Holy Spirit required to undertake full communicant membership of the Church. When a person has been baptised in infancy, the baptismal vows made on that person's behalf by parents and god-parents are re-made by the candidate at Confirmation.'
It is a clever definition because it affirms the proceedure without casting doubt on any other dedication or commitment made by the candidate. If someone comes to confirmation already baptised as an adult the rite simply expresses the desire that the person should receive all that is necessary from the Holy Spirit to proceed as an adult member of the Church to which they now belong.
It is akin to being prayed for before going on a long journey or undertaking a difficult task. Who could doubt that belonging to the Church of England, whilst retaining your sanity, is a long journey and a difficult task?