In principle there should be consistent price convergence across the market so that you enter your data for a quote and prices end up being similar within a reasonable % variation from mainstream companies.
My observation is that since the 1990s the prices quoted vary dramatically by more than high double digit %.
If you use a price comparison site (Compare the Market etc.) you can get a load of quotes for the same car based on the same parameters in terms of NCD protection / voluntary excess / usage / drivers / mods / convictions / annual mileage / value / etc. Things like compulsory excess / breakdown cover / legal protection / courtesy car / personal accident cover will vary from policy to policy, which you do need to take into account.
But anyway I got quotes today for my R129 SL500 - these ranged from £182.80 to (drum roll) ... £3,500