In view of your previous good character you may be able to persuade the Magistrates to deal with the matter by way of a Conditional Discharge. If not, as the guidelines suggest, you will probably receive a fine at "Band C". This is equivalent to one and a half week's net income but it will be reduced by a third (to one week's income) if you plead guilty. You will also pay around £85 in prosecution costs, a "Victim Surcharge of 10% of the fine (minimum £20, maximum £120) and a "Criminal Court Charge" of £150.
If you plead not guilty but are found guilty following a trial you wil not be eligible for the disscount on the fine, the prosecution costs will be around £600 and the Criminal Court charge goes up to £520.