You pay in advance, and so there is a catch up for last month when you owe from 19 July to 31 July.
You then have the cost for August. What you should see is the bill from 1 August to 26 August [i.e. a month after you cancelled, the exact day will be around then, maybe a day or so earlier].
The bill seems to have gone over that. As above from what you say it should be up to 26th August or within a day or so of that. If so it seems that you are being billed over that period. When was your bill dated? It may be that it was produced before your cancellation, and thus you will have a small credit next month.
Does your box show the cancellation? Under apps and then subscriptions, mine showed the days left for the subscription when I cancelled Sky Sports. If not, then there is an issue. If it does show that I would expect a few days credit next month, but just five or so days, and no issue.