Pension type benefits allow you to set the contribution as either a percentage or currency but not both - it's one or the other.
In the meantime, a suitable workaround could be to track the monetary/currency type contributions in the 'Deductions' table in the Payroll category (for £ being contributed by the EE towards their pension) or the 'Entitlement' table, again in the Payroll category (for £ given to their pension from the Company)