We would like to setup a Vigor 2820 as just a dumb modem. Our customer has had a Juniper firewall setup that is configured with PPPoE dial out and so needs a device in front of it.

My understanding is that if you enable PPPoE pass through for LAN, any device can use the Vigor as an ASDL circuit out but use it's own credentials to initiate a connection. Is this correct and if so can anyone direct me to how this is done. I've read elsewhere on the forums that UK based ISP's don't support PPPoE pass through?

The theory is correct. The ADSL modem in the router can passthrough to the juniper firewall. The 3.3 firmware supports this. The option you are looking for is a checkbox in the WAN1 configuration screen - PPPoE passthrough or PPPoE bridge