Minimum daily rate in Zurich you would accept

[Brussels Slumdog]

Only swiss companies can invoice Swiss companies for salaries

Just curious, if I were to drop for a 5day training in .ch would I still have to do that through payroll company or is it just for 'disguised employees'?

The problem is that you need a swiss employer or payroll company to invoice the client. If the end client has another european subsidiary then they could send you to CH to provide the training

A payroll company maybe willing to invoice the end client less commision and no questions asked.
Whether you declare one weeks work in the UK is up to You.