I don't think its helpful to contemplate how the salary of the office staff compares with the amount of money being offered - the £100 suggested here certainly has doubtful bearing on the costs incurred getting to that stage.
What is much more helpful is to consider what possible further costs and what possible income lies ahead!
The Company is being given a simple choice: accept £100 now with no further expenditure on evidence gathering, briefing legal professionals and incurring legal costs, OR, disregard the offer and continue incurring professional costs in the hope of being awarded a much more substantial fine by the Court?
When you consider the costs of legal work that lie ahead, then maybe much of a £1000 fine is attributable to the evidence gathering and professional costs (as well as the admin mentioned by others above), so the net benefit to the Company of proceeding might only be around £100, with the risk (albeit small) that the prosecution will fail.
In that light, perhaps £100 would be attractive. Whether a Company might adopt a policy favouring prosecution or favouring acceptance of 'offers' is another matter.