Sometimes I just use whatever is lying around in the house, but quite like Rose’s lime juice cordial: https://www.sainsburys.co.uk/gol-ui/product/roses-lime-juice-cordial-1l
Most of supermarket squash/cordial offerings here in the UK has had the sugar taken out and replaced with a sweetener, so you have to read the labels. It probably doesn’t matter overall but it feels counter productive to make a high carb flask and use a cordial containing artificial sweeteners 
I’ve been doing sugar, water, salt. Both my bottles and my refillable gel bottles. Roughly 80g per bottle and 60g per gel.
People find it weird but honestly if your gut can handle it why not. Mixing ratios of different carb sources might increase digestion but i much prefer the simplicity of this.
how did I know…hahah (Lived in Houghton for a year and a half)
I’ll share my method for those interested. It’s a very simple solution, but it works for me.
I buy pure fructose and glucose in bulk (5kg bags). I mix them up in a 2:1 (glucose:fructose) ratio. This is convenient, because I just buy 2 glucose bags and 1 fructose bag. I have thought about doing 1:0.8 but haven’t tested this yet.
For every 80gr of mix, I add 1g of tablesalt.
I use this mixture in my bottles up to 100gr in 500ml. This dissolves by just shaking the bottle for about 30 seconds.
For long races, this same mix is used to make gels. I dissolve up to 300gr of sugar into a 250ml running flask:
The sugar needs heat to dissolve properly into only 150ml of water. With the sugar added, this takes up the whole 250ml of the flask.
This video is the same process I use to dissolve the sugar into a “gel”:
A couple of weekends ago, I did an XCM that took 7 hours. I did 100gr in my first bottle, second bottle on the bike was plain water. In the back pockets I had 2 gel flasks with 300gr of carbs. So 700gr in total for 100gr/h. Stopped in 2 aid station to quickly fill the bottles with water again (or i could run a camelback with plain water).
I’ve never had gut issues doing this, and it’s cheap. (I calculated 700gr mix costs me about 3 euros in supplies, excluding buying the soft flasks)
My brother has tried the same thing with plain table sugar, but experienced some issues. The next time, he used the pure glucose and fructose aswell, and had no issues.