On certain Netgear ones I know its possible to define the SNR margins manually, and a few other settings. Why would you want to limit your sync rate out of interest, surely its best just to let the modem negotiate the fastest stable connection possible.
ADSL24 and some other providers still sell fixed line packages, have a look here:
http://adsl24.co.uk/broadband_home.php
Also if you are on a long line or having sync issues I would recomend getting somthing that has a Broadcom chipset in it, they have a good reputation for getting the best sync on a bad line - somthing I would agree with having just upgraded my old DrayTek 2600 to a Netgear DG834N I'm now syncing more reliably at 1.6Mbit instead of 1Mbit and being unreliable.