Looking at the emulators for MR200 and MR400 (here
respectively) it seems they don't
support L2TP - only IPSec VPN client. AFAIK AAISP don't allow other VPN protocols (PPTP, OpenVPN & IPSec), so you would need to subscribe to another VPN provider such as NordVPN and use their UK server for a IPSec connection. However this is likely to be slower than AAISP's L2TP service.
You're better off going for the Billion 4500NZ which definitely supports L2TP VPN (more info on pages 103-104 in user manual)
FluidOne FTTPoD 330/30