They will, but it will be a lot less than $1.8m a dose. When you're buying on behalf of 68million people you get a lot of purchasing power in the negotiations. Now a doctor can just say "you need x treatment go and get it" instead of battling with an insurance company (who also won't pay list price (but it will be more than the NHS pays) however they will bill you as if it cost them 1.8million.
Though its a rarely used drug so you are talking about dozens of kids per year. Also Japan which is twice the size of UK was only able to negotiate it down to $1.5M.
The amount paid by the NHS to Novartis per treatment is confidential - there is no charge to the patient. Japan also provides the same drug free of charge through their public health service, but they have said they negotiated the price per dose to be $1.5M with Novartis.