I agree with all that has been said above. One extra detail may be useful; where changes behind the modem do affect your speed, indirectly related to DLM.
All FTTC ISP's use the DLM described above; based on cabinet->modem stats, and ultimately setting cabinet->modem sync speed.
Several FTTC ISP's also have a higher level layer of BRAS/IP profile that limits the rate the exchange will send your data to the cabinet.
The IP Profile is set when the exchange->router PPPoE connection is established; unless your package says otherwise this is just a fraction below the sync speed to allow for overheads.
Often, when DLM forces a resync with new sync speed, the PPPoE connection is broken, and so the IP/BRAS profile is automatically set appropriately.
However, quite often, the low level resync is fast enough that the higher level connection does not notice and is not broken.
This can lead to a BRAS mismatch, where the sync speed and IP Profile do not
The speed you get will be based on the lower of the sync speed and profile.
If you think that your speed is too low and the DLM may be going to increase it, it is worth disconnecting and reconnecting the modem first thing each morning.
Best done by the web interface, but you can power cycle or reset the router.
Do it early morning as DLM usually schedules speed increases overnight.
Moved (with trepidation turned relief) to BT Infinity 2 for upload speed. Happy BE user for several years.