Technically they are. Sky don't like reducing competitors prices. But, as an example you may be able to add BT Sports at full price but get a 'reduction' on your Sky channel package. It helps if you are in a Virgin Media street, you can play one against the other. You can't cheat though because...
You might be able to get a deal if you have SKY Sports as part of your package. Failing that, just ring and ask, if you don't get anywhere give them 30 days notice of cancelling your package and then wait for them to call/write to you with a better deal.