I was talking about this yesterday with my nephew's fiancee, who is a nurse. Her experience is that such shortages are not rare but are, in most cases, connected with the renegotiation of contracts with the drug companies. Acting in the interests of their shareholders, the companies usually try to increase the price or at least maintain them, whereas the NHS is usually keen to reduce it, especially on drugs where the development cost have already be compensated for. So you get a stand off during which the drug supply is halted. She has experienced this many times.

Brexit is a factor because it impacts the exchange rate and hence the cost of any drug prices in euros or dollars. It is also a factor because drug companies for entirely sensible business reasons are wondering when the most profitable time to sign a new contract will be: now or in a few months time. And this works at other levels: the companies that supply the raw materials to make drugs go through a similar process.