DD:
RYR is not U.K. based neither do they operate G-registered aircraft.
The employer therefore has no obligation to accept their [BALPA's] dealings.
Forget aircraft states of registration for a moment. In fact forget aircraft. Ryanair employs a substantial number of individuals in the UK, therefore is subject to UK employment law, which provides for statuary union recognition if necessary.
Also, you mention BALPA company cars. By pure consistence, I've just read on their website that they're selling one. An 8 year old Vauxhall Astra with an ignition problem if you're interested