The president is scheduled to visit Buckingham Palace in October.

The White House has made clear it regards the carriage procession down the Mall as an essential element of the itinerary for the visit currently planned for the second week of October, according to officials.
...
Barack Obama chose to spare his hosts the task of securing the journey from Horse Guards Parade to Buckingham Palace when he was welcomed by the Queen in 2011. In a slight change of usual protocol President Obama arrived at the palace in an armoured motorcade.
...
“The vehicle which carries the president of the United States is a spectacular vehicle. It is designed to withstand a massive attack like a low-level rocket grenade. If he’s in that vehicle he is incredibly well protected and on top of that it can travel at enormous speed. If he is in a golden coach being dragged up the Mall by a couple of horses, the risk factor is dramatically increased.

He does love gold and extra expense out of others pockets is no concern of his.