Spoke to Apple CS via the phone, asked whether the accessories could prevent me getting the iPad on the 28th. They said yes, but if you call customer services you can get them to split it into two orders for you, so that you're guaranteed to get the iPad on the 28th (or before if it's anything like the US).
Quoted from the "What does my order status message mean?" section of the Apple Store help pages:
Partially Shipped:
Part of your order has left our warehouse. We often send orders in multiple shipments so that you'll have your products as quickly as possible. The rest of your order will ship as soon as it's ready.