Seed potato growing is regional, and mostly in the north and Scotland. So you don't need a natiowide drought to impact on supplies, rather some problem that covers the seed producing areas.
Our association had no problem obtaining rocket this year, and some on line seed suppliers (Suttons) for example are still selling but DT Brown are showing Rocket as "sold out", and Marshalls as "not available". So not really clear to me, one way or the other.