Register your phone number here at the Telephone Preference Service.
http://www.tpsonline.org.uk/tps/ All companies selling via the phone are supposed to regularly vet their database against this list and breach of those rules can lead to them being fined by Oftel. You can use this service for domestic and business numbers. I've used it for years and get no calls at home a probably 2 or 3 per week at work.
There's also a Fax Preference Service which works weel too.