I have a 2003 model converted to LPG

how do I get a certificate of conformity 

Hello welcome mel You can't I have a 3.0 with lpg only 2007 onwards is ulez compliance having a 2003 with lpg won't get a certificate

You get the Certificate of Conformity from here: https://japaninspection.org/certificate-of-conformity-coc/

 

Once you have that, you provide it to the DVLA and they will update your V5 to have ULEZ compliance. Having an LPG conversion doesn't make a difference either way. 

The vehicle to old for ulez registration anything before 2007 

No it isn't. All Alphards are Euro 4 compliant, but earlier vehicles need the certificate of conformity from Japan to prove it to the DVLA, who will then update the information to make it ULEZ compliant. 

 

Loads of people have done it, but one example is here in the forum