I worked behind a bar for the student summer vacations in 1971 and 1972. I can remember for certain that, in 1971, 20 Embassy No 1 cost 26p. With slightly less certainty, I think that they'd gone up to 29p by 1972.
(For anyone who's interested, in 1971 the price of a pint of mild was 10p. Bitter was 11p. Lager? Don't be daft. We didn' sell that 'funny foreign beer'!).