In the UK, traditionally light switch wiring fed the live wire down to the switch from the light and the switched returned a 'switched live' back to power the light. In this case there was no neutral available at the light switch itself which complicates powering any smarts inside the switch. It can be done but this relies on using a trickle current across the switch even when it is off.
More recent builds in the UK do have the lighting wiring arranged to give a neutral connection at the light switch even though it is not used for dumb switches.
The Sonoff units require the neutral connection.
If your zWave unit also used a neutral connection then it is likely that you will be OK. However, there are some zWave units that use trickle current powering, as does the LightwaveRF scheme.