The original cable, while an imperial size, is close to 16 mm2 and for 99% of Mini Builds, that would be adequate.
I use single insulated 25 mm2 in most builds, but rather than buying 'battery cable', I use a flex as it runs in much neater and is nicer to work with, though you'll probably want to get bell mouth lugs to be sure they'll be easier to get over the cable stands.
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I buy mine by the drum (from RS Components), but wander down to your local welding supplies, they'll likely have it by the metre on the shelf.
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