Just like you have a choice over which vendor you pick for your copy of Apache, in the UK you have a choice over which electricity supplier supplies your electricity. In both cases the end product is the same no matter where you get it. I've got the ability to choose my supplier.
I wanted to switch gas and electricity suppliers after working out I could save over £300 a year with a different company. I was going to switch a long time ago, but with this being a new house it took the current electricity company over 8 months just to *find* which meter was supplying me (they didn't believe me when I told them my meter serial number and came out to check it on three separate occasions!). Anyway this company really isn't happy I'm switching and they want to make it as hard as possible to do so - by sending letters dated 13th December but that mysteriously don't arrive until the 17th telling me I have until the 18th to give them £13 or they'll stop the transfer, having customer service reps who are really happy and helpful up until the point they see you're leaving then making it difficult and painful. Customer service, even when you're leaving a company, is important.
Rather than being hostile they could ask me why I'm leaving, wish me luck, and act efficiently, so that if the new company doesn't work out I'd be happy to switch back to them. Leave me with a positive lasting impression. How about sending me a "We're sorry you're leaving" card in the post perhaps with some tick boxes "what could we do to win you back"? Thats the most valuable marketing data a company could hope for.