Android Question I'm sure the answer is no but I'll ask...is it possible to change the wifi hotspot channel?

I don't have a "yes/no" answer for you but I have a question...
You ask about changing the pi's WiFi chanel when in AP mode (I think it is possible, but not sure if it's easily exposed), and then you say that it reverts to GB when it connect TO your smartphone, so it is no longer in AP mode, let's call it Master, but in normal mode, let's call it Slave, which is normal to me, since it gets all needed info from the smartphone, that is in this case, the Master.

First of all - I cannot be at all authorative on this because I have not tried to do what you are trying to do, but a couple of sentences in you post surprise me.

. . . 5Ghz, presumably because it isn't really a legal channel in the UK.

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Are you sure? I live in the UK and 5GHz appears to be a perfectly valid WiFi band - it would be difficult to find a router that does not support it. My router from BT certainly does and it is several years old. And anyway, how does your Paspberry Pi know which country it is in?

. . . connecting Raspberry Pis to my phone hotspot

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You do not say which Raspberry Pi model you are using. Most current models support 2.4GHz and 5GHz wi-fi but some (Raspberry Pi Model B and some Zero versions, I think) do not. But I might be mistaken; not really my subject. Consider me friendly rather than knowlegeable.

I don't think I was quite specific enough. I am using a Pi 5. I am saying that channel 161 in the 5Ghz band is only legal in the UK for indoor use and is reserved for things like radar. I believe equipment wanting to use that channel is supposed to check to see if there is any traffic on it before using it. When my phone (very rarely) chooses channel 36 in the 5Ghz band the Pi connects to it fine but the phone usually opts for channel 161 which is legal in some countries but it is restricted in the UK. Channel 161 was actually only opened up for restricted use in the last few years.
I have tried setting my Pi to a different country and while it is able to scan and see hotspots on that channel, as soon as it tries to connect it switches its country region to "GB" and is no longer able to even scan and detect those frequencies. That is why I am asking if there was anything tricksy that can be done. It looks like the Pi is picking up the region code from the broadcast data from the phone and acknowledging the "GB" region and switching its legitimate frequencies to "GB". If that makes sense.
My windows laptop has no issue connecting to channel 161.

Cable Guy. I am only referring to trying to connect to the Pi to my phone where the Pi is the client/slave and the phone is the AP/hotspot/master. If I had the option for the other way round my life would be so much easier, but sadly...