You have had a number of answers which I think will be helpful.
My two pennyworth would be.
1. If you have a business line with BT, should you have a problem they will sort it out more quickly. (Mobiles and call diversion have somewhat reduced this advantage)
2. It does depend a little on what sort of work you are doing, but with an 80/20 connection you can shift very large amounts of data very quickly, so unless there are a lot of people on the end of the connection needing to shift lots of data at the same time, it should be fast enough.
3. During normal working hours, the bandwidth gobblers are at school, so contention with others outside your workplace should not be too much of a problem.