Santander/Aviva make changing from Yearly DD to Monthly bizarrely difficult - why?

Wonka_2

Devongardener said:

I think you need to have some respect for the people here who are trying to help you.  You didn’t say in your original post that you only wanted replies from people with knowledge of the insurance industry, so it’s inevitable that you will receive advice from a wide range of experience.

Some of the replies are from knowledgeable people with (significant) experience of the industry but the OP's chosen to ignore/diss them

dunstonh

I'll restate the original question, in a way which possibly makes the comparison easier to understand for some people:

Policy A (for £400 p.a.) is buildings insurance, and has been yearly DD. Renewal time has come and I asked for it to be changed to monthly payments. Santander/Aviva say this is not possible without getting the Underwriters involved

Policy B (for £1200 p.a.) is car insurance, and has been yearly DD. Renewal time has come and I asked for it to be changed to monthly payments. Aviva say this is perfectly possible without getting the Underwriters involved, and do this within 2 minutes.

I refer you back to my previous answer on this but I will dumb it down for you even more.

Some policies are available on annual premium basis only.   They may not have a credit facility as default.   
Other in-force policies may not be able to be changed between payment methods without manual intervention.

The only variables in the above are Buildings vs. Car, and Santander using Aviva vs Aviva direct. 
They are not the only variables.   
Q: From your practical knowledge of the workings of the insurance industry, which of the two variables results in the different outcome (and why?)
Neither. You are looking at the wrong thing.