I don’t know if it still runs, but I used to go to the Tesco wine fair in Victoria. £10 for a four hour session of every single wine they sell. Used to get absolutely battered 
More recently I’ve gone to the annual wine festival at the Oxford Union, more expensive, more civilised, but just as pissed.
Probably won’t be one for a while now but try googling for them, particularly in London when things get back to normal