Blocked pipes whats the best way to go about this?

[Illuminate]

Gurgling sound from basin when shower running, and water seems to be backing up shower when basin running a while. Shower running slow and when using a basic plunger on it backing up a lot of waste. Bottle trap cleaned out and part of the pipe work.
Should i install a bottle trap with antisyphon valve.
I was thinking to buy a better plunger such as a pressure gun version.

What is the best way to go about this?

 

[WC1]

Basic plunger best.
Block air from entering / leaving pipework while plunging.
Plenty hot water also.
Keep plunging.
Pressure gun a bit of a gimmick imo as chemicals also.