Mini wine bottles (20-25cl) are usually £2 each at ASDA. They run an offer where if you buy a mini, you get a voucher for the cost of the mini, off the 75cl bottle.
A lot of mini's are currently rolled back to £1 but when you go through the till you still get a £2 voucher off the 75cl bottle!
Tried this successfully on Jacobs Creek Shiraz Rose and Stowells Italian Sparkling Rose. I noticed a few others rolled back too but I didn't have the time or inclination to keep going back for 1 more bottle!