I have been trying to find another result for the ruling party that matches the Labour result in Henley and I have found 3 which almost match. The 1992 General Election position is noted and then the by-election result.
Barnsley East 1996
A strong Labour seat - the Tories go from 2nd place and 14% vote share to 3rd place, 7% vote share with 1299 votes. Still ahead of the 4th and 5th placed candidates. The Tories get wiped out at the 1997 General Election.
A strong Labour Welsh seat - the Tories go from 2nd place and 14% vote share to 4th place, 3.9% vote share with 913 votes. Plaid moves ahead of the Tories. The Tories got wiped out in Wales at the 1997 General Election.
Monklands east 1994
A strong Labour Scottish seat - The Tories go from 3rd place and 16% vote share to 4th place, 2.3% vote share with 799 votes. SNP moves ahead of the Tories. The Tories got wiped out in Scotland at the 1997 General Election.
So this leads me to the conclusion that this is not totally unprecedented for a ruling party to drop so low, although to fall behind the BNP is a bit of a howler. However when this happened to the Tories as the last unpopular ruling party, the subsequent General election lead to annihilation.
My prediction - the end of Labour in England apart from maybe 100 urban seats. SNP destroy Labour in Scotland and tactical voting by Lib Dem/Tories and Plaid see Labour off the map in Wales.
Let's hope Labour do not change leaders.