Try phoning Ribble to ask?
The frames will come in in big batches from Taiwan or wherever, and it may mean that the last batch is sold out and the next batch isn't due for a couple of months, or even until next autumn.
I've come across other sites marking products as unavailable when what they mean is out of stock, and will be for a while. If they left it active, they'd have upset customers asking why it was taking so long, and there could be currency fluctuations that would affect their willingness to take long lead orders.