Are there any Wi-Fi modules for the Arduino Uno that may connect to an external Wi-Fi Antenna?
There are variants of ESP8266-based modules which include a connection for an external antenna. The ESP-02, ESP-05 and ESP-07 modules all feature a U.FL socket.

Effectively, they are an ESP8266 mounted on a carrier board with a U.FL socket.

If you already have some code to suit an ESP8266-based module, this will avoid having to rewrite for a different unit.