London Weather- late August

I'd say 2018 more typical but then who's to say 2019 will be typical? February certainly wasn't. This week we've had temps break 20 C for the first time but roll back a year and we had significant snowfall across much of the country.
I wouldn't count it as essential.

Or 2016 when Aug 23-27 hit 86, 90, 81, 79 and 79?

I think the odds favor not-hot, but those odds won't do you much good if you are miserable.

On two recent trips that fell in September or mostly in September, I opted for a less expensive lodging that had no AC, and it worked out fine. This year my London time is all in the month of August, and I'll be at an air conditioned Premier Inn.

Length of stay might be worth considering. Two nights, maybe three--I might roll the dice if the savings were substantial. Longer than that and I, personally, wouldn't take the risk.

If you take a chance and lose, my tips for surviving are to ask early for a fan (they may not have enough for every room) and take tepid showers.

Go for the A/C. There would be very few dollars saved between Premier Inn with A/C and a B&B or hotel without A/C. And would it be worth ruining your visit for a small savings?

If you will be staying in S. Kensington, I can send you a PM and tell you where not to stay. A major chain that promises air conditioning but has a small A/C unit in each room that functions for only 18 minutes and shuts off.
It's on a timer which is controlled by the front desk. It won't come on for the rest of the night unless you get up and push the reset button. Then it will run for......18 minutes and shut off while you are asleep.

I say it's better to have it and not need it than need it and not have it - just get a place with AC. If it's hot you're set...if it's not it won't matter anyway. The weather in London is so unpredictable! NO way to tell what it will be especially in August.

Go for the A/C. I find hotels to be hot places anyway, particularly in built up areas. There's no way of knowing what late August is going to be this year. Last summer was quite a hot one and I would certainly have wanted A/C then but this year might be a total washout, who knows.

Avoid the term "air cooling" that is not A/C like we know it. In Europe last summer in July it was warm and everywhere was uncomfortable. Even with temps in the high 70's, the cathedrals, palaces were stifling with the crowds. I literally saw condensation on the paintings in St. Petersburg. I hated that we didn't get to see some of the palaces - in Stockholm (I think,,,) the royal apartments were so warm they were warning people not to go in. Not to go down another road, but it was fabulous after a warm day out coming back to a cool room!