To achieve this feature try to search module on connect which allows to add surcharge to subtotal. You will have one default rule with discount. Your second rule will have surcharge (your additional 5 pounds). I tried to find some one, but all of them not free. You can check their demo pages.
For example one extension can add surcharge like this:
Sure you can rename this field. And this surcharge adding automatically, like price rule. I'm not sure it is configured also there.
Or maybe you'd like to add two rules in one row?
For example following extension:
4) This can be used for additional shipping fee or import duty or surcharge or service charge or any other additional fee
5) Additional fees can be added as fixed amount or percentage
I think this can suite you.
Configure your Flat rate shipping method:
Your method name will be 'Freeshiping' (not flat rate)
Put fixed amount per order.