The GARDENA gardening expert
Your assumption that the ants were washed off the roof is probably right. It is worth checking that it really was the runoff that swept the ants down the gutters and into the water butt. This then begs the question of why the ants were on the roof or in the gutters in the first place and where their nest(s) might be. I received your question in mid-April, just when these insects become more active — for instance, climbing up plant stems to establish colonies of aphids on them (for "milking"). So why would they be scaling your summerhouse? Are they trying to reach host plants? For example, ants frequently "farm" aphids on elders. Or could it be that the ants have nested somewhere in your summerhouse, in the wall or under the roof for instance? If so, and if the ants are causing problems or could cause damage, it is probably best to call a pest controller. These suggestions should enable you to find a way of preventing more ants finding their way into your water butt.
Insects that land on your garden and plants with water from your butt will not do any harm. Dead ants become "fertiliser", and any surviving ants will simply die in your flowerbeds because they will be unable either to find the scent trail to lead them back to their nest or to start a new colony.