I've made a number of bags for my bicycles, mostly my vintage steel road bikes, so not a Brompton. Pictures of your current bag and where it is mounted would be helpful in offering suggestions.
This is the latest set I made for a '70s Gitane and to match the remade [MENTION=73614]rhm[/MENTION] custom saddle. I repurpose purses.

It's part of my winter "stay sane" therapy.