An additional £1,000 for a 2TB drive is crazy though, rapidly takes the new Mac mini from a good price to a nuts price
You'll just have to be careful to match the enclosure to the ports on the system. The base-model M6 Mac mini still uses Thunderbolt 4, so a USB4v2 enclosure would be wasted.
You might be able to boot from the internal drive, install/upgrade the external drive, then boot from it.
Compared to the prices of storage today, when people are presumably buying the product, it's actually a bad price.