As others have mentioned, I would suggest really having a good think about whether you genuinely “cannot afford” £120 or whether, on reflection, it’s more that you feel that sum is more than you wish to pay or you consider it “unfair”.
If you truly, genuinely cannot muster up the £120 by borrowing/begging from friends or family, selling/pawning personal items, etc., can you offer to pay in instalments? They’re not obliged to accept this offer and you’d want to be looking at no more than three months or so.
Should the matter go to court, and result in a conviction, which it almost certainly will, you’ll be looking at a fine in the vicinity of 50% of a week’s wages/benefits, plus £150-200 in costs, victim surcharges, and compensation. This can be paid by way of an instalment order (e.g. £5/£10 a week) but will cost much more in the long run.
This won’t go away so it’s best to engage and talk in terms of what you can do/offer.