If your connection speed on 40/10 was below 40 then changing to 55/10 wouldn't give you any faster speed. It's a change to the speed limit for your line but won't alter the current line capability or the VDSL2 technologies used.
If your current VDSL2 connection speed is below the guaranteed minimum then in theory you can raise a fault with BT and they should investigate/fix it.
BT don't advertise general minimum speeds, they advertise an 'up to' maximum such as 38, 52 or 76.
They give a specific estimate for a line which nornally includes an estimated range and a guaranteed minimum. These are for connection speeds and not data download speeds which will be lower particularly at peak times.
The HomeHub5 stats also show an estimate of what your line is capable of in it's current condition.