I just called PlusNET and they said that once a customer is regraded to 8Mbps they automatically get a maximum of 448kbps upload speed. As usual, whether you get 8Mbps and 448kbps depends on the usual environmental factors.
The only PlusNET package that supports the full upload speed is Business, at £41 per month (inc VAT). Their Teleworker package is only £16 per month (inc VAT) but also only has 1GB per month bandwidth included (as with the Small Business package. All of the Business packages are on 20:1 contention ratio, which can make a big difference to the usual 50:1 if lots of other people in your area are big broadband users.