Victoria British tells you when online ordering whether a part is in stock, I have not had to get a refund from TRF, but have had a couple things on back order, my three experiences with back orders from TRF have involved months, whereas Moss VB and BPNW have been days or weeks at most.
Pure conjecture, but I think the others mostly order more parts as soon as they run out, I think TRF must wait until they have a batch of the part or parts to order from that supplier.
That being said I have ordered most of my TR250 parts from TRF, as my car needs everything they have had something for sale in their weekend specials pretty much every weekend that I can use, and that has saved me major bucks.
As far as time of dleivery, a lot of that is simply where you live, for me VB is much much faster than anybody else. But right now as the car won't be back on the road for some time and there is plenty to do, time of delivery is not an issue right now.
For all the suppliers I am amazed at how well stocked they are, it is often faster and easier these days to get parts for your LBC than it is many times for the more obscure parts (outside the routine maintenance parts) for your modern iron.