Invoicing system / invoiceplane / CRM - discussion and feedback

ERPNext

This looks like the most promising option so far, see the ERPNext for Services page. But 1.6k issues but the install looks OK and it has a basic command line interface.

Video on how to set it up for different VAT rules for different countries:

Loads more on their YouTube video channel.

Here they differentiate themselves from systems such as Odoo and Dolibarr:

100% Open Source

Unlike other open source business platforms, ERPNext is fully Open Source, because unlike them, we donโ€™t think sustainability and open source live at odds.

Most open source platforms in the ERP / CRM / E-Commerce space are open core . These communities makes money from closed extensions and services to monetize their efforts. We believe this is not only wrong, but not sustainable.

https://erpnext.org/open-source

One the list of ERPNext Service Providers there is one based in the UK: