Add ability for free users to see shared model with more than 20 tables

We don't plan such possibility. It is an element of our current business model. 

In that case, what about a change to the business model to allow users to view only for a pound or something similar, that's attributed to the creator's account?

I would struggle to use the "share" functionality as a work tool if I can't show the diagram to anyone that wishes to peruse it without them paying a membership fee...

You can use public link functionality instead of sharing model in such case. You can set it on model details screen.