If you are using electric showers then you will always have very poor flow from those due to their nature. (They restrict flow as you can only get so much flow from 7-9kW showers)
If you want any real advice then you need to supply more information:
- current water flow per minute in litres...
it is a grove in the middle so that the pipe stops half way through when you push it before soldering. Cut below or above those couplers (depending how long your tap flexes are) and use either isolation valves are 15mm compression coupler and remove nut and olive other end and install your flexi...
I would go with Nest from the two. More refined and nicer interface. Honeywell T6 will be cheaper, but nowhere near as nice as nest and software used to play up on mine. No problems since I changed it to Nest
Pull the silver cover out, remove little basket / trap thingy inside. You will see that the waste has little grooves or teeth. See if you can turn it clockwise. If you can then it means it loose. This happens a lot.
you also need an S flange fitted to the cylinder so that pumped water is not taken from the top of the cylinder to avoid air getting in. Best fitted next to the cylinder and fed directly to the shower