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130% means they get to claim 130% of the cost as an expense in their tax return.

For example, if their income was £200, they spent £100 on fibre installation, and there were no other expenses:

Normally their profit would be £100, and they would have a tax bill of £19. Profit after tax would be £81.

With the superdeduction, their taxable profit would be £70, and they would have a tax bill of £13.30. Profit after tax would be £86.70.