Using the "Contact Us" wizard to report a speed issue, you are adviced as follows:
"In order for BT to investigate further you must leave your ADSL hardware powered-up and plugged in. Also insure that you disable any firewalls that might be in place. This will allow BT's engineers to examine your setup. Failure to do so will introduce delays."
Is it wise / safe to disable your firewalls for an undetermined length of time?
I see no reason why security should be compromised just for BT. In any case hardware firewalls make no different to your physical ADSL connection and I see no reason why they need to be disabled anyway.