I'm not sure what you want to achieve but be aware that if you brew any kit at a lesser volume than intended you increase the bitterness in proportion. In this case brewing to 23 litres compared to 40 litres will roughly double the bitterness. That might not worry you.
So as an example if you want to brew your Harvest stout to about the bitterness intended by Youngs and with about the same ABV as the Muntons stout I suggest you use 2 x 1lb jars of Holland and Barrett Malt Extract and 500g of Golden Syrup, or any combination of similar sugars equivalent to 1.5kg of LME.
And If you want to see what other people have done with a 1.5kg stout kit have a look through the Wilko Stout review here
https://www.thehomebrewforum.co.uk/threads/wilkos-dark-velvet-stout.28049/Coopers stout reviews (more LME) are here
https://www.thehomebrewforum.co.uk/threads/coopers-irish-stout-review.17818/https://www.thehomebrewforum.co.uk/threads/coopers-original-stout-review.17817/