Whatever you do please don't forget to make sure you have access to the valve - they need charging up from time to time and should be checked at the annual service.
If I read one of your posts correctly it appears that the PRV is connected both ends? Is this right? If so it is wrongly installed as one of the connections should be open to a tundish.
An 18 litre vessel is about right, plenty on the internet but do make sure you get one for potable water (usually white or blue in colour) and NOT a heating vessel (usually red)
You need a bigger cylinder to take advantage of off peak power , and a modern one - perhaps think about unvented (mains pressure) for a decent shower, too. If you live alone I would suggest a 210 litre.
There's no reason why you can't fit a pump, but these chaps are correct - you will also need a mechanical safeguard like a thermal safety valve and discharge/quench arrangement that works without power.
Here's what you do. Pick a tap connected to the cold mains (kitchen sink) and turn it on. That's approximately what an unvented hot out will give you. Now turn on a second mains tap (hose tap?) and see what happens to the flow. That's what happens when two hot outlets are on. It will give a...
I might be making an assumption here but you mention that "but did run a tap upstairs. The flow rate was 15l per minute"
Does that mean he checked an upstairs tap? Almost certainly this would be connected to a feed tank above it, so the true measure is at a cold mains outlet like the kitchen...
Yes, you would get more "hot water" because you would be using less from the cylinder and more from the cold to mix it down. These units are protected against overheat so adjust as you feel suits you best.
If this has been mentioned sorry, but the only time you are going to get 20 or 25 litres from a combi is if there is storage somewhere. A lot of combis have storage built in. Once the storage has been used you are back to a more typical flow and temperature rise.