I would like to implement free shipping for orders over a set value, e.g. shipping becomes free when the customer adds £100 or more to the shopping card. This is all straight forward and I don't require help setting it up
Shipping and discount codes are an unfortunate pain point. This is because a customer enters their shipping address first, and then the option to add a coupon code is the page after this. With your current configuration, a customer will still be able to use a coupon code after qualifying for free shipping.
Something that may be a decent workaround is creating two coupons: one for free shipping, and another for a discount on the order (the coupon you already have). Because you can't stack coupon codes with Shopify, this leaves your customers the option of either getting free shipping over £100 or using your current discount code.
I hope this helps!