In my experience this is a normal occurance in the sense all that you are doing is transferring a volume of soapy water from one location to another and as ling as you have agitation you will get bubbles, all I can suggest if you aren't prepared to wait for the bubbles to dissipate by themselves is to sprinkle some salt on them.

Try taking the waste pipe below water level in gulley

It depends on 2 things. The depth and flow of your external trap, and the amount of bubbles you use. Also, you could consider installing a longer vertical pipe, so that it delivers the waste straight into the trap, below water level. I think maybe if the water is falling out of the pipe then it's creating new bubbles in the trap as it lands in the soapy waste water, same as what happens from tap to bathwater. If it went direct into the trap water, it may not do this. Think of putting the shower hose below water level when filling the bath - that would mean no bubbles. Same applies, if you get my meaning.
Perhaps you can put an additional plastic cover on the external grid, if you have one, to mask the bubbles, as a quick solution.

Change ur drain grill to a larger diameter or change your soap products