Currently the swap has no positive value to the financial institution, so there is no credit risk (i.e., counterparty risk) to the financial institution.
If the swap had a positive value to the bank, then the credit equivalent amount under Basel I would have been (Amount of Swap positive value to the bank + 0.5% of the principal amount of the swap).
For instance, suppose the principal amount was £100 million and the current value of the swap to the financial institution was £2 million. In this case, based on Table 12.2, and given it is a four-year swap, the add-on amount is 0.5% of the principal so that the credit equivalent amount is £2 million + (0.5% * £100 million) = £2.5 million.
The credit equivalent amount for an off-balance-sheet item is multiplied by the risk weight for the counterparty in order to calculate the risk-weighted assets.