Working from home has never been considered a 'business purpose' by ISPs as far as I'm aware.
So why would you look at a Business product (before complaining about the charges applicable) ?
This doesn't justify the additional cost regardless.
It's probably more to do with "market forces" and how some businesses would ignore a "too cheap" supplier, as well, on the grounds that the low cost means they are unable to provide a good service.
TalkTalk, under their business branding Opal (now defunct) used to offer 'unlimited' broadband for half price at a fiver a month for the first year, on condition one had their phone service.
There's no longer that low cost broadband from TalkTalk Business, and while an ISP wants to attract new customers, and market share, I seriously doubt the sales and support teams would be able to cope if tens of thousands of small business customers switched in the course of 2-3 months. I know there's definitely a separate business sales team, but I don't know offhand how support is handled (ie whether the same or different staff, as for residential).
As always, PN would take on extra staff, but there'd be a delay before they were up to speed on the business products and be able to assist fully. Have seen newspaper ad for Plus.Net aimed at smaller businesses, so know they are looking to get switchers, but perhaps not as fast as pricing the same as residential users would cause...
Also, I believe there may be a 'business' or 'residential' flag determined by postcode and while most times a business would want a business line, someone in the Business Sales team did provide a client of mine (a small charity on a military base) with service on residential rates, so they seem to have some flexibility there anyway.
If you run a business, have a second ISP and backup web hosting...