It seems very likely from your description of the incident that, in the event you are convicted, the Magistrates will consider that a fine is appropriate.
However, there is more to consider than simply the cost. At the moment you are of “good character”. A conviction will ruin that and it can affect different people in different ways (in employment, education, etc.) and you should take this into account.
It may well be worth getting a solicitor’s opinion of the strength of the evidence against you. You should have been provided with copies of the statements against you and to get an opinion should not cost too much. Also bear in mind that if a trial is held the witnesses will have to give evidence in court under oath and they may not be too keen to do so.
Nobody on here can properly advise you how to proceed, but you should think carefully before you admit to doing something you believe you did not do, or maybe end up being tried without having taken advice.