The rate of recession being proportional to distance is a consequence of a conventional explosion. Some things blow apart faster than others. So it seeems reasonable that X being three times further away than Y, must be travelling three times faster than Y because they both started in the same place at the same time. This is what Hubble observed.
But this is true wherever you are, on the planet Earth or on a remote galaxy. Wherever you go, it looks like YOU are the starting-point.
You can demonstrate this by drawing a straight line on a piece of paper with points marked every inch (say). Then, pick any point and call it the origin. Then, mark the other points as having 10 units of speed for every inch they are away from the origin. You have something like ... +30, +20, +10, 0, -10, -20 ... The rate of recession is proportional to the distance from the origin.
Now, pick any other point where you think we might be (say, on the planet earth at point +20). Now calculate the apparant rates of recession relative to us on Earth. You will see that they are exactly the same as for the origin. WHATEVER point you choose, it will always appear the same. There is no difference from one point to another.
In other words, there is nothing to differentiate the origin from all the rest. A bit like being in an infinite forest - how do you find the middle? All the trees look the same, wherever you go it always looks the same.
On the large scale, everything is moving away from us in straight lines - if they were not, then it would not support the idea of a big bang from a singularity.
I think the experts assume that expansion is the same everywhere, but mainly because there is no reason or evidence to suppose otherwise. Any other assumption is a guess.
Hoping this clarifies my ideas. I can't blame anyone else!