Interesting question. From my experience, small dealerships can often offer a more personalised and pleasant experience than big chains, but they have much smaller build allocations and so it can take a lot longer to source a car from them. Big or small dealers, it often depends on the individual sales person and how much they are prepared to go the extra mile, if at all.
We ordered our current BMW from a massive dealership, but I know the sales person (as I had ordered a private car from them a few years earlier) and even though it was a Motability order we got nearly £1k in discounts off the AP and factory options. Furthermore, they secured a cancelled private order slot for an X1 M-Sport and switched it to us for our Motability X1 X-Line, so we got the car in less than 4 months. Needless to say, when we get our next BMW, whether it is through Motability of privately, we will use them again.
The wildly different lead times that users of this forum report are almost always down to build slot allocations (or lack of them) and sales people not being upfront about that. Manufacturers don’t build cars quicker for one dealer over another, so if a dealership takes 3 months longer to get a car than another it’s invariably because they didn’t have any build slot allocations and ‘sat’ on the order for 3 months. Very few, if any, dealers will ever own up to this and always blame the manufacturer to deflect the heat off them!