Full NCD, 2 accidents in last 5 years: one my fault in a car park nearly 4 years ago, no damage to my car (Disco 3, so no claim), damaged indicator to other car; one other driver's fault, MG5 written off last September. In the following, all the quotes are for 11k miles pa, spouse as named driver, no points, includes no claims protection, all the usuals (no breakdown cover), and with a £500 voluntary excess, monthly payments (so can knock at least around £60 off for paying it all up front).
I'd been with the RAC for the MG5 (and Disco 3) and their underwriters wouldn't quote on an S5 (my choice to replace the MG5) without a reg number last October/November. In early December I tried again: no reg number was up around the £1k mark, but put a reg number in and then a couple of their underwriters quoted something up around the mid to high £700 (IIRC). That, and other things taking precedence in personal life, temporarily stopped me getting my MG5 replacement straight away.
Fast forward to early January and I was still primarily thinking of getting the S5 when the S6 deals started being advertised, and so I widened my choice. In getting quotes for the S6 the RAC underwriters wouldn't quote for it with or without a reg number. Using a comparison site, an initial no reg number illustrative quote for the S6 Trophy threw up a range from £650 (Admiral) to about £1100. A few days later, having decided to get the S6 I used the same comparison site with the reg number and got around £550 with the AA (which didn't appear in the no-reg quote). And if I'd wanted to wait another roughly 10-15 days to pick the car up I could have got even cheaper insurance, but how much cheaper I didn't explore.