It'll be controlled via the cabinet, but can be limited via the quality/length of your line. So either 40/2 sync or 80/20 sync maximum dependant on package, but if your line supports less than 40 then changing to the 80/20 won't make any difference to the download speed, but your upload will likely be better. Which ever is the lower (line or product) will always be the limiting factor.
Openreach sell four products for FTTC, 40/2, 40/10, 55/10 and 80/20. TT only sell 40/2 and 80/20.