I do ovens occasionally, its hard work but you can charge arround £18 to £20 per hour. average Oven takes me arround 2 hrs I charge arround £35 to £45 per oven. what ever system the frachisors offer it still boils down to scrubbing. My hi tech equipment is as follows
1 x large bucket the type u see in B&Q for storage
1 x collection of scrubbers
1x tool kit for removing doors, glass etc
as far as chemicals go, on a slightly soiled oven Brillo oven cleaner and soak grills in a strong detergent. or baked on grime, Caustic gel and soak grills in Caustic soda (I know I will get slated for using this, but its not as bad as some think and if you take care its not a problem) read the instructions for this and use at your own risk. and remember to catch all waste water and disspose of in the appropriate way.
The only advantage as I see it with a francise is the heated tank, if I wanted to do more of this type of work I am sure it wouldn,t take much to knock one up. ( just ask your local metal merchant to knock you up a stainless steel tank with a lid £200 max and fit a 12v caravan water heater element and a drain tap
They you go, a franchise would have charged £3,000 plus for that info.
on a serious note take care using caustic soda, you need gloves and gogles when using the stuff, I will only use as a week mixture and never ad water to caustic soda allways the other way round and never add caustic soda to boiling water, read up on it first before using.
hope this helps Phil