Aha, my specialist subject and sadly the answer is 'not really'.
All mainstream squash has sweeteners in. You used to be able to get blackcurrant and apple/mango flavours but I've just looked on a couple of supermarket websites and they're not there - we only really buy squash in the summer and I remember DP said he couldn't find it when he last tried to buy it.
The posher brands don't have sweeteners in, so if you're trying to find it in a supermarket look for the ones that cost more money for a smaller bottle, eg:
https://www.ocado.com/products/rocks-blackcurrant-squash/585250011
It's incredibly difficult to buy soft drinks without sweeteners in pubs/restaurants etc in the UK.
I thought I had it nailed when I decided that my drink of choice when I wasn't drinking alcohol would be an orange juice and soda. But this is far harder than you'd ever imagine and tends to go like this:
An orange juice and soda please.
Orange and lemonade?
No, fresh orange juice and soda.
Me, watches as bar person starts to put sweetener filled orange cordial in a glass. Hi, sorry, no, fresh orange juice and soda, while pointing to the carton in the fridge behind the bar.
Bar person is apparently lost in their own world and carries on making my drink.
Hands it to me and I tell them it's not what I asked for and I very politely, while screaming inside, explain in words of one syllable and a lot of pointing that I would like fresh orange juice (pointing at the carton that I can see, I'm not expecting them to start feeding actual oranges into a juicer) and soda water, not lemonade.
If I'm lucky, I will get it with a lot of eyerolling, but half the time it's a seemingly impossible task and I wonder if I'm the only person in the world who doesn't happily drink the disgusting tasting artificial shite, which this thread shows, is not the case.
I once gave up at a pub that did table service when the third drink they brought me was still cordial and soda, again pointing at exactly what I wanted in the menu, because I like a lot of fluid with my meal so needed something to drink and fortunately for me, it's only the taste I object to (I don't have a particularly refined palate but to me all artificial sweeteners taste disgusting), they don't make me ill, like they do a lot of people.
Although I've now learned that Appletise is just apple juice and sparkling water so while I'd prefer orange juice and soda, if the pub has Appletise, it's a lot easier to ask for that.
The problem is that a lot of people lie about what food is in the UK and you have to watch them like a hawk if you're unreasonable enough to not go along with it. So they call squash juice and don't pick up that fresh orange juice is something different. See also trying avoid anything except butter on your bread/sandwich.
To me, only pure butter is butter, which is what I want, but I've had the 'butter' version of the above experience when watching servers put crappy spread on my toast/sandwich in cafes etc. If you ask them if it is butter, they will say yes, because in their minds, butter is any old yellow fatty crap that you spread on bread.