Same with EHIC - can't use after end of this year. But don't throw yours away just yet! As far as I know there's no new agreement yet but that's up to the gov.
With unforeseen treatment under EHIC we actually have waiver agreements with several countries - Denmark, Estonia, Norway, Finland, Hungary and Malta. In other countries I believe actual costs of treatment are claimed back from each to each.
As to planned, pre-agreed treatment as described in the OP, perhaps Rowan or someone working in that sort of area can explain more. I believe it's down to individual Trusts how much they charge and how many beds they allocate for such treatment.