My 8Mb ADSL from BT rarely gives me more than 350Kb upstream, so it won't support more than 3 VoIP calls at once.
AFAIK there are no UK broadband providers offering free international VoIP.
BT offer Broadband Talk, which includes a free geographic number, and cheap intl calls. You could just DIY it, using Vonage, SIPgate etc, but it won't be free.