I have read somewhere that companies can charge for a single if a return isn't used, they have your card details, and it can be expensive. The ferry companies do the same, when they have their £1 day trips they're not supposed to be used by, for example, people returning to their overseas address.
I recently priced a trip to the US from Heathrow and from Manchester. The Manchester trip was £20 cheaper, even though it meant flying down the Heathrow on the shuttle and catching the same flight across the Atlantic!