Is there confusion from actual pastry process rather than the Kenwood? I'm scared of sounding a twat in case you know this or I have always been totally misinformed but.
Cold butter in pastry leaves lumps as it doesn't "melt/coat" into the flour. These small lumps create little hot pockets when baking that makes your pastry flaky/crispy.
This is why I use suet in my savoury pies (like Ma Dragon does> to make it crispy as it stays solid and break down on baking making a lovely crispy pie top.
Slices of room temperature butter are fine. You get a nice uniform pastry that is all.
If previously you have always rubbed by hand till you get uniform crumbs than you melted the butter through anyway with your body temp.
Are you happy with the pastry you are making? What do you want to change.