No, not a tactical error, just plain speaking.
There are probably negotiating channels available, remember some hostages from other nations have been freed, presumably after the payment of a ransom. However as the UK will not pay ransoms, those channels are not of use.
ISIS/IS/ISIL or whatever they are called this week, seem to want to get the US tangled up in this with boots on the ground. That would give them the opportunity to portray the US (and presumably the UK if we went in as well) intervention as a direct conflict between Islam and the West.
Ultimately to defeat them there will have to be boots on the ground, but some of those boots have to come from Sunni and Shia states as well as Western states, otherwise it will look like a Christian West fighting Islam, even if that version of Islam is a perverse interpretation of the Quran.