Hi @patrickhaond,
Thank you for reaching out.
Just to reiterate on what community member @hemant-ahir has already shared.
To change the starting invoice number in WooCommerce, you would need to use the help of a plugin, as this feature is not available in WooCommerce by default.
One of the plugins you can use is Sequential Order Numbers Pro for WooCommerce. This plugin allows your store to have sequential, rather than random, order numbers. Additionally, you can customize your order numbers by setting a starting number, order number length, and custom prefixes and suffixes. Instead of your order numbers looking like 15, 16, 37, and so on, they could now be WT-01000-UK, WT-01001-UK, or whatever format you prefer!
We offer a 30-day refund policy which you can take advantage of, allowing you to test the extension, and make sure that it works the way you need.
I hope this helps! If you have any other questions, please feel free to ask.