There are not enough details presented here to form an opinion on whether the assets are sufficient. Just because use of assets was not "ticked" does not mean you cannot use them.
What IS clear is that your affidavit of support was not correctly completed, in a way that the assets could be considered. You entered income that cannot be used.
If your assets are liquid, and are well over three times the income requirement, they can and will be considered in London. You mention "income" from the assets. That would only count if it shows on the US Citizen's Federal tax return, consistently being above the income threshold. Is that the case? If it is, then your current income comes from the "total income" line of the 1040. You can then state and document the value of the liquid assets, in the asset section.
Armed with that information (exactly as stated above) do you still think you qualify?