Of the ones I've tried, you can only do that if the .uk itself registered and dropping. If the .uk is available, but can't be registered because the .co.uk has ROR, you can't backorder it.
My understanding is that if the .co.uk drops, it loses ROR and the .uk immediately becomes free for all. That's the scenario where I want to grab the .uk, and the catch services I've tried (dropped and ukbackorder) don't seem to cover this. Or did I completely misunderstand how the ROR works?