On funding,
AFAIK there's more than enough in the unallocated headroom (an amount set aside and carried forward with Treasury agreement to fund priorities from the whiteboard of unfunded wishlist items, as and when they're afforded priority) to fund an up front purchase of 6 or so.
But running them would need to be accommodated from a flatlined or shrinking running costs budget, so might mean a bit of deckchair shuffling...
Anyway, until such time as the Treasury reneges and snatches the unallocated funds (which is possible but which would be politically and presentationally difficult, especially given the current Tory backbench sentiment on defence and HMG's tiny majority), there IS funding available for this acquisition. At least for the capital part of it.