Adding an isolating valve to old plastic 15mm piping - what brand to use?

[Question]

I have a sink and want to add an isolating valve for the taps.  See photos.  The 15mm plastic pipes are at least 20 years old or probably 30- 40 years old.   Should I get a

 

(a) plastic isolating valve and not worry about what brand it is.  In other words, different brands are compatible with each other.

 

(b) plastic isolating value of a particular brand (if so, please tell me what brand)

 

(c) copper isolating valve, and if so do I get a pushfit or compression or either.

 

(d) do not touch it.  (the plastic pipes go under the floor). 

 

Grateful for any advice. 

Edited April 22, 2023 by Question

[Question]

I think I can answer my own question. 

 

The cap of the T junction has the distinctive polyplumb brand markings.  So I'll get a polyplumb isolator.

[Iceverge]

I did away with all the isolating valves when I replumbed our house. 

 

Considering how irregularly they'll be needed the inconvenience of turning off the mains for an hour while I fix a tap really wasn't worth the extra point of failure or cost  in my opinion. 

[Temp]

Just make sure to fit pipe inserts.

[Nickfromwales]

Yup. Polyplumb. Some of these exact fittings still available on eBay. 
 

[Question]

Thanks very much for your comments.  This came up, as I have to change a washer in a tap.   

 

I don't think the plastic piping is polyplumb, as there are no markings on the pipes.  So there is already a mix of different brands, the T junction being polyplumb. The whole thing has been in place and fine for 20 years. The polyplumb T junction was probably added by a plumber who was extending the water for an extension.   So I'll just leave it alone and switch off the water from the main to change the washer, like Iceverge suggested. 

 

[Nickfromwales]

KISS wins again!