Well last week our local Tesco had a few English butterheads, and some sweet Gems, plus the (disgusting IMO) Icebergs. This week there were plenty of USA origin Romaine lettuces, and some Icebergs (also apparently from the USA, 'tho I didn't notice). No policy posted on purchasing. So, a shortage of the usual Spanish suspects certainly. Broccoli (Calabrese) was still plentiful.
As Furan says, we import a lot from Murcia (maybe 90%) and they've had a complete growing cycle wiped out by floods and frost. So, a 6 week wait for the next supply coming onstream.