You are obviously hunting for an answer that gives you the loophole you are looking for. So, if you are comfortable with the UK consultancy take, go with it.
If you "choose" to exclude product design and installation, are you willing to have such exclusions CLEARLY documented? ISO 9001:2015 mandates you do that. Are you comfortable with the idea of your customers reading that you don't have a quality system that includes product design and installation? They (the customers) might not appreciate that.
Organizations come in all sizes, shapes and degrees of complexity, so for a multinational organization, with multiple divisions, business units, sites all over the place they MUST be very careful when determining the boundaries of their respective systems.
And don't forget the fundamental distinction between scope of the system and scope of certification. When it comes to certification, yes, you can limit the scope of certification.