To work maybe a fan blowing air from inside towards an open window with you in between is enough. I'm using that in Italy with similar temperatures this month. Don't let direct sunlight get in or it will become too hot and monitor the temperature of your hardware.

However my memories is that UK houses are built for a colder climate than even the north of Italy (which in winter is colder than London) so you might need an AC anyway. I'm using one in the night to sleep with closed windows (city noise outside and darkness.)