Isn’t that a 3.5t anyway? Gonna be weighing 800kgs + ? Google says 2800 - 3250kgs for 1cx
Your straps, especially at the back, the angle is far too big between the trailer bed and the strap, that’s why it’s moving. aim for more of a 45° angle at the most.
Or get them to get you some of these
Silverline 911664 Car Transporter Tie-Down Set Alloy & Steel Wheels Rated 2000kg Capacity 4000kg amazon.co.uk/dp/B01AM8HDJ0/ … BCbCQP4D8Z