Cardiff it wasn't a comment on the P50 but a comment on the article that made a business case for the fire service into providing the P50 that I don't think is a realistic comparison and would like to know where they get the data from. You are quite right that not many schools, if any, apart from the odd boiler room where a risk assessment may prove that a DP is necessary but that wasn't the point - the point I was making was that one supplier is making a business case comparing DP and even using a DP extinguisher that is not even available in the P50 version.
As for the rest of your post; the provision of CO2 is unnecessary in most situations that I find them and I often advise clients not to take this option so once again that cannot really be used as a comparison; CO2 is for protecting electrical risks where it is needed to penetrate into the inner workings to extinguish the fire or where the voltage is high and the P50 won't be able to replace them for this task. So we're looking at a provision of like for like extinguishers over say 21 years of a company this will require an initial purchase of a P50 (27A) at ?180 inc VAT and two refurbishments at ?288 = ?468 working out at ?18.72 P/A. If we're doing the comparison with a 9l foam (27A) at ?50 per extinguisher at ?50 installed plus the ?10 a year that you're quoting then that works out at 5 extinguishers (forget about 5/10 refilling etc just get new ones) at ?250 plus ?210 worth of servicing at your maximum price of ?10 and hey presto - its cheaper. This is a very simplistic example in order for you to understand and make a point however it far more complicated than the only worked example that I could find which appears to be specifically designed to fool people into thinking the P50 is way cheaper.
My point however is about costing - the fire service involved will not have costed out their support and it would be interesting to see their full analysis and any business case put together; who is delivering, commissioning and training the on site personnel or are they merely buying them from another seller who carries out all of these tasks and then taking a commission? If they are supplying all of these services are they including their insurances, support services, training etc in their own costs? The business model is simplistic and operates on a similar basis to private health care - let the NHS train up all of your staff then poach them when they are fully trained so you avoid many of the costs. To be fair it may give many of the older guys a job until they draw their pension at 84 years old. The FS are providing these through "commercial operation of the service?s new trading arm, Vital Fire Solutions (VFS) Ltd ? a sister company to its Community Interest Company" - its in the linked article.
Extra space is easy to find for most businesses - just tell them that BS 5306 is guidance only and that they don't have to comply but must comply with FSO (law) and get a risk assessment - take into consideration the actual risk and reduce the number of extinguishers by around half and write this on the risk assessment and they get loads of extra space and reduced maintenance costs. Simples.