OK, to accomplish this, you should add another column to your cart for itemType.
in the add to cart buttons, set the value of the item type column to 1 for the cheaper items and 2 from the more expensive items.
you then need to create 2 shipping rules.
the first will add an £8 if an item with itemType = 2 is in the cart.
the second will add £3.50 if an itemType = 1 is in the cart, but no itemType = 2
itemType 2
Triggerrs:
"Number Of Items with a specific column value"
If the Number of items where [itemType] has a value of [2] is [>] than a value of [0]
Calculation:
flat rate charge of 8
itemType 1 - no itemType 2
Triggerrs:
"Number Of Items with a specific column value"
If the Number of items where [itemType] has a value of [1] is [>] than a value of [0]
AND
"Number Of Items with a specific column value"
If the Number of items where [itemType] has a value of [2] is [<] than a value of [1]
Calculation:
flat rate charge of 3.50