fitting a 3-way T valve, what flexible hose to order?

[abo_hosni]

Hi,
I'm new to UK plumbing system and got lost as don't know names/diameters/availability of the parts I need to do the job.

I guess I'd need to remove one of the copper fittings and order a flexible fitting of some sort and diameter. Don't worry about the length I would get a suitable one.

I'd like to insert one of these T-valves inline so that I can connect a shower head to one of its 20mm ends

Here are the two T-valves I got here (I only need to insert one inline)

And here it's where I'd like to insert the valve.

What are the names of the parts/fittings I would need.

Much appreciated

 

[ShaunCorbs]

What’s the flexible hose / shower head for ?

 

[abo_hosni]

What’s the flexible hose / shower head for ?

It's for a hand-held bidet with a 20mm hose connection

 

[ShaunCorbs]

due to uk water regs you cant install one of those off cold mains water due to the risk of backflow, needs to be done properly with a break tank with weir, shower booster pump, under sink water heater and tmv3