The seven exhaust and primaries can be kept shiny with not much effort, using Autosol metal polish (from Halfords). It almost does not matter how bad they get before you polish them, but it does vary the amount of time a hand polish job takes to make them shine again. I think I polish mine about 2 - 3 times a year for a maximum of about 30 mins each time.
I have never used the Car Plan products being advertised, I use a proper Snow Foam lance from Ultimate Finish and Blit Hamber Auto Foam. Snow Foam is supposed to be used to clean the car so it is clean enough to be washed with a buckets and mitts. This all helps to minimise the tiny fine scratches (called marring) which you can see on most cars (even new) which spoils the deep shine. My tin top has a nice polish level, and my snow foam cleans the car to such an extent that most of the time thats all thats needed. While waiting for the snow foam to do its job you can see the dirt flowing off. I very much doubt a product designed to be used on a garden hose will perform as well.
For a quick shine, try Tough coat from Auto Finesse. Its a trigger spray synthetic way which takes about 10mins to polish a tin top.
Finally I wouldn't use a pressure washer on my Seven, so hence I won't Snow Foam it, so for me that leaves a problem of how to get it clean enough for a hand wash. Auto Finesse to the rescue again on this, because they have a pre wash product called Citrus Power which is a hand spray on product that lifts dirt.
Hope this helps