The £150 offer is this
http://www.freesatfromsky.co.uk/
Which gets you the receiver/dish/installation/Freesat card for CH4 and FIVE, no contract and no obligatory phone line connection is required
The Dixons offer (if still available) for £75 gets you receiver/dish/installation and a subscription to 4 Mixes for 4 months - plus a phone line connection is required or you can pay an extra £25 and not connect to the phone line (total £100)
If you don't wish to continue with subscription channels they must be cancelled 30 days before the end of the contract - in my experience not 31 days or 29 days but exactly 30 days. The card will continue to get you CH4 and FIVE.
If you don't cancel you'll be charged £19/month for the 4 Mixes