Hi Ceejay and welcome to the forum. We are a rescue friendly forum so the discussion of breeding is forbidden please see the forum rules for this. I do not know if a UK adder is able to eat a guinea pig however if you research their natural diet if their prey are similar in size then yes they could, if they usually eat harvest mice then its very unlikely as a general rule with snakes they can only eat a meat the width of their head so in general only large snakes would be capable of eating an adult guinea pig, babies however will be more at risk.