Hi,

I'm assuming you've not done any Advanced Rider training, if you have then IAMSurety do free business use with their standard policies for IAM members and will beat any quote. They might do something for non-members. If you can tolerate TfL for a few months it might be your cheapest option to go through the process of doing the test and have additional qualifications to add credibilty for your business.

Hope it helps,
Penfold