is air conditioning in London hotels turned off for fall?

Most Premier Inns have air conditioning in their regular rooms. However, I don't believe I've ever been in one as late as October, so I don't know whether the a/c will still be running then. (My guess would be yes, but that you're unlikely to need it.)

That's interesting because their website for the ones I have looked at in London indicate A/C only in Premier Plus.

You'd have to ask the individual hotel but I don't remember it being turned off in the few I've stayed in colder months.

Premier Plus has nothing to do with A/C. Premier plus adds a real coffee machine, a mini-fridge, a more updated rooms with plugs, and I believe automatic premium internet.

As have never found the premium wifi to be high speed in any PI, that is probably not a big deal.

Which London locations say a/c only in premier plus rooms?

No they don’t turn it off but you probably won’t be able to chill the room right down if you like it really cold. There is some sort of limit on it.

Maybe it is a recent change? As requested:County Hall, and most of the ones I looked at IN LONDON
Premier Plus double
Our enhanced room design. Includes Ultimate Wi-Fi, coffee machine, mini-fridge, bedside USB ports, iron, upgraded workspace,
Air Conditioning

It’s usually not that hot that ac is needed.

I'm not aware that in locations which don't have air conditioning generally, that they have installed air conditioning specifically in Premier Plus rooms, but at County Hall, for instance, all rooms have A/C- https://www.premierinn.com/gb/en/hotels/england/greater-london/london/london-county-hall.html?cid=KNC_Brn|_B_UK_UK_Eng_Enc_Brand%20Destinations_LO_London_EX&mckv=s5ujc192t_dc|pcrid||kword|premier%20inn%20county%20hall|match|e|plid||pgrid|1209462896745794|ptaid|kwd-75591507275209:loc-188&ef_id=9ad1af8fecc41ecc1c6fcb97684a0fea:G:s&s_kwcid=AL!9693!10!75591465813363!75591507275209&msclkid=9ad1af8fecc41ecc1c6fcb97684a0fea
(click hotel facilities at top left- it says A/C and then Premier Rooms) so all rooms have A/C. But you can't get it down to icy cold, there are upper and lower temperature limits.

If a PI doesn't have A/C- like Preston North- it says so in the 'Important Information' below the list of room rates.

I've stayed at the PI County Hall and can confirm that the regular rooms have a/c.

The Premier Inn website is quite poor by the standards of major hotel chains. It often leaves the viewer with questions rather than answers. And don't waste your time trying to call. Occasionally an email will be answered, but expect to have to dig to find an email address. (The best source is the individual hotel's Facebook page, but you'll have to start by Googling for that.)

A/C in London in October? I would be more concerned about heating 😅.
Joke aside: days are short in October, so even if there were a hot spell, nighttime temperatures will be lower.
And I say "joke", but last year, in warmer Paris, I had to turn the heating on for a 10-day cold spell between late Sep and mid Oct.

Thank you all so much.

We have been to London a few times and, in fact, stayed in the Premier Inn County Hall about 10 years ago when we returned to London for the night before our flight out. However, it was 10 years ago and things change. With the climate changes we are experiencing, it seemed prudent to make sure that A/C was an option.

As an aside, part of my concern has been prompted by this winter in New England. In the greater Boston area, there really has been no snow to speak of, my granddaughters are disappointed by no sledding and no snowmen and no snow days. I am a little concerned about just how hot it will be in New Englad this summer and fall and thus my concern about A/C in October in London.
Taking acraven's advice, I did find the County Hall fb page and have contacted them with my question.
I am grateful for everyone's thoughtful comments and advice.

A/C units in Oct will turn to heat pump/ventilation only mode (heating).

Most large hotels in UK have computerised building management systems which will set the mode dependant on a combination outside/inside temperature (mainly outside) - It will depend on the indivdual organistions energy management policy where the limits are set. But as a rule thumb it is likely to be.

Below freezing > 14/16: Heating mode

14/16 > 22/23 Free cooling with heating as required

23/24 and above: Cooling mode.

Because of the general low levels of humidity in the UK free cooling is widely used instead of mechanical cooling, even at higher temperatures.

Often hotels (particulary budget ones) will have units that look A/C units but in fact they are either fan coil units (which only heat) or air/ground source heat pumps, the former are much more common, but the later are gaining tractions due to Government incentives and grants.