Even though the calls come from abroad, they're trying to sell products and services within the UK, so it's likely that the companies behind the calls are UK-based. This means that they should still be observing the TPS rules.
For a short while, it might be worth answering some of these calls in order to find out the names of the companies behind them. You'll then have more information to hand when you submit your TPS complaint form(s), here:
http://www.tpsonline.org.uk/cgi-bin/tps_complaints.pl
If you're with BT, you might also want to consider their 'choose to refuse' service. (�3.35 per month). The relevant webpage is one that won't accept a direct link but you can get to it from here:
http://www.bt.com
(At Home: View All Services > Home Phone Services: Calling Features > Advanced Features > Choose to Refuse).
Chris