When I bought my Leon 2 years ago I asked Brokers4cars just that. They said they'd probably retain it, but I should have it within 6 months. Also I wouldn't be the First Registered Keeper. Those two things put me right off.
Went with DTD (second time I've used them) and it is essentially exactly the same as walking into a dealership and placing an order. V5 arrived about 3 days after I picked the car up.
I can only speak from personal experience having bought 3 cars through them and my youngest daughter one, I have been the first registered keeper in all cases and have received my V5 direct within 4 weeks of taking delivery, I think it's very much around what arrangements they have with which franchised dealers they are working with at the time so def worth asking.
I have a friend who ordered a Citroen DS through them about a year ago and he didn't get his V5 for 4 months BUT, the way they have to work that now to stay within the law is that he had to be the first registered keeper so his name was on the V5, just the address was not his, then the supplier did a change of address at the 4 month point and he received the V5 in the post from DVLC, and Yes he was the first and only registered keep on the document.......... but again I agree it is a question that has to be asked because dependant on what car make you are purchasing will depend on the arrangement any broker has with their supplier.
On my last order with them I paid a £500 deposit to the broker once I had approved and confirmed the order and options, got a receipt back same day, I then paid the balance to the finance company I think (I got it on PCP) the day before it was delivered,, I paid a £185 broker fee but that included delivery to my drive.