The Brimstone missile is mainly used against vehicles or where it's smaller warhead means less chance of collateral damage. Otherwise, the Paveway IV seems to be the weapon of choice.
It would also seem that much of the anti-ISIS activity carried out by the RAF has been in Northern Iraq in support of Iraqi ground forces.
Whilst the Reaper has seemed to be more in use over Syria and, of course, uses the Hellfire missile.