How much does a ribena cost in the UK?

depends what type you mean and where you want it from.

all 2L

ASDA:

bottles: £4.89 or 2 for £6 (on 21/04/2012)

cartons: £2.69

SAINSBURYS:

bottles only: £4.99

TESCO:

bottles: £4.89

cartons: £2.69