The bayonet acts like the sv
As for reusing old ones aslong as he tested it it’s one of them is it needed providing it’s not leaking no is it good practice yes
Never heard of them
As for the bar mixer looks like a stnd cheap bar mixer that’s around £200
Bristan zing £80 lol
Best thing I could advise is there a local city plumbing showroom there iflo brand is pretty good for the cost stick with 6mm minimum glass thickness
If your going concealed...
Easy thing to test
Run the shower then run the kitchen sink fault should happen if yes your on a shared main
Two options live with it or get your own water supply installed eg from the water suppliers main underground to your house etc
You don’t have enough water flow and pressure to have both / two on at the same time let’s say each body jet takes 5lpm you want 6 min that’s 30lpm you have half that
Also a system that you want won’t be cheap best part of 1k just for the shower at minimum and your only options are grohe or...
Sounds normal
Boiler has to go through it’s sequence of opteration normally around 15-20 seconds then has to heat itself up as it’s cold
Hence the quicker operation when heating as everything is warm / hot
Can’t see why providing you use it on and off as you currently do
I can see there logic behind not running it as if on constant it would drop the store temp / waste heat through natural Heatloss of the pipework