Depends on your definitions. The "United Kingdom of Great Britain and Ireland" has only existed since 1922. The earliest date you could really count would be the Act of Union with Scotland back in 1707, which set up a United Kingdom Parliament for England, Wales and Scotland, so 1710 would be the date I think.

Of course they were run a bit different in those days. The first one resembling a modern one in any way would have been 1832 with the Reform Act, and even then the electorate was pretty small.