there are certain things you aren't allowed to buy with voucher eg cigarettes tobacco mobile phone top up stamps first stage baby milk products instore concessions (whatever that means )lottery and fuel
It does say on the voucher what isn't included so you won't be caught out at the till if you spend £20.50p and something isn't included....though I find they aren't too picky if you smile..
Not all Co-ops are created equal ... they are still fiercely independent within the overall umbrella of the name - hence different coupons/dates/offers.
I worked for CWS (a long, long time ago) and the infighting between the various component parts of the Co-operative movement made The Borgias look like The Woodentops ...