I had a brodex e compact in a vauxhall astravan for a number of years, that was installed with the tanksafe brackets and as i had modified the chassis by driling great big holes in it, it was classed as a modified vehicle so it was more to insure.
Now if you strap your tank down using racthet straps and the load posts provided, its not a modification as your transporting stuff as per design. Really you only need to worry if you are over your payload. Also, drive slower ans yoy minimize your risks.