Re: Because for example...
>Business use of residential/consumer broadband service
This matter was well covered in the recent discussion of article "GREEDY SKY ADMITS: WE CRIPPLED BROADBAND WITH TOO MANY USERS" ( http://www.theregister.co.uk/2013/01/22/sky_broadband_adding_customers_first_capacity_after/ )
Basically it seems that only Sky explicitly disallow commercial and business usage. (See thread starting http://forums.theregister.co.uk/forum/containing/1700793 )
However, as pointed out by others the SLA you get on a residential contract is minimal. If your line is important to you then you have two choices:
1. Upgrade to a Business Broadband for Home workers/ small businesses package, which isn't that much more than some residential deals - and if you are in business you'll be able to expense it, reducing the actual price slightly.
2. Stay with a residential contract but invest in backup, like a mobile broadband dongle etc. that works for your location. (actually I would advise doing this even if you go for business broadband, as I've found that things tend to fail just when you actually need to use then and out-of-normal-business-hours.)