CEDR does give you the option to upload documents, but in this case I doubt much is needed, other than a dated copy of your first complaint to show that the first complaint was 8 weeks ago, alternatively BA's deadlock letter. Otherwise a brief, brief summary of the key points and PNR should be sufficient. BA will need to prove you did not start your journey in London, it's not for you to prove that indeed London was your start point. I suspect BA will resolve and pay this claim before the claim is sent to the adjudicator. In any case, before that happens, BA's reply will be circulated to you on their complaint management system (massively better than BA's own system) so you can correct any wrong statements that they may make.