Data globalisation -> Russia wins
The proles may have voted for Brexit but that wont stop the powers that be needlessly globalising everything. That we the people aren't consulted about how our own data is stored demonstrates how our "democracy" works. If we are consulted, it will be after they've implemented it so that reversing the decision will be costly and disruptive.
Even if cloud hosting costs are cheaper elsewhere, the data transmissions costs must be taken into account. It's probably worth paying more if the extra money is fed back into the local economy.
If the Russians are capable of tapping transatlantic cables, data security is at risk. In the event that they cut our communication links (or if they are cut by mistake or natural causes), the NHS wont have reliable access to patient data.