There will not be an additional excess to use another bodyshop mate, I am sure of that (I work in the insurance industry).
Generally insurer approved bodyshops are not the best IMO as the insurer will try and get the labour rate down as much as possible. They try to get you to use them by telling you that dont need an estimate, no inspection etc, convience for you basically.
If I was you I would find a decent local bodyshop.
Simon