You can offer to pay the fixed penalty and hope that the prosecution is withdrawn but there's no legal obligation upon the local authority to do so. They might state that they'll only stop the prosecution if you'll pay their legal expenses (which they'll already have incurred) on top of the fixed penalty but, far more likely, they'll simply go ahead with the prosecution.
Your fine will be based upon your income but, as a giude, expect it to be in the order of £150, plus £20 'victim surcharge' and perhaps £80 in costs. (i.e. you'll probably have to pay around £250, together with getting, or adding to, a criminal record).