At this stage, pre-orders need to be placed separately to any other items in your basket. You can simply place your pre-order, then go back and place your order for the rest of the items in your basket.
If you're trying to use a discount code across both, we'd recommend using this on the pre-order, then dropping an email to hello@spoke-london.com to let us know - we'll be happy to share a second code for your regular order (provided the conditions are met).