The NHS is a whole other argument – quite simply companies should only be able to claim up to the value of a GOS eye exam or the minimum value they charge privately.
As such if a company offers a free private exam for any reason then during that period they should not be able to claim the NHS fee. If they only charge £10 then they should only be allowed to claim £10. Its not significantly different to how the spectacle voucher system works anyway.
Back to topic – a very difficult task and I do not see how a new union could be set up in a feasible manner. However it may be possible for an existing union such as Unison that would be interested in supporting the sector, after all we are providers of NHS services.