Here is what I remember:

The book starts out in Washington.  A terrorist plot is discovered by a female agent, FBI?, and is brought to the appropriate gov agency. The director knows that this is an extremist op and calls in a recently retired/parted CIA or some other black ops agent.  He has had his fill and is reluctant to take the mission but makes a deal that  this really will be his last mission.  He grudgingly accepts the mission and they are sent undercover to Europe/England to track down and kill the terrorist quietly. I think it was sanctioned by the President. He is an “off the books” guy and is not tied down by following the law while the female agent is by the books. She slowly learns what it takes to accomplish their mission and slowly warms to the guy. They have to use fake names, fake credits, and stay off the grid while tracking the terrorist, as I think the terrorists found out about it and began tracking them. I remember they try to buy some guns from some Eastern European gangsters and get into a shootout in a London garage when the deal goes bad. He has to use his contacts to get around and to track terrorists. I know they rented a seedy hotel room and he went out to get food and a burner phone and someone came to attack them while she was alone. They had to deal with the underworld trying to get info on the terrorist. I think the book ends when they catch-up with the terrorist attack around the London eye(ferris wheel?). The terrorist may have been a suicide bomber with a dirty bomb, but can’t remember for sure..

For some reason I thought it was written by a female author but I have searched every female action/suspense writer and have not found it.  All I listen to is spy/action novels and the title must have popped out when scanning the titles but it wasn’t obvious.