This is how other constituencies, in a similar position to Mid Bedfordshire, have voted in past by-elections:
Kensington and Chelsea (November 1999): Con HOLD
Beaconsfield (May 1982): Con HOLD
Southend East (March 1980): Con HOLD
Hertfordshire South West (December 1979): Con HOLD
Of all of the by-elections Sunak will face, or is facing, Mid Bedfordshire will allow him to see if Labour is winning the commuter belt. If Labour gain this seat, then he will wait to the last possible moment to call an election, if the Lib Dems win he will call it in May 2024, and if he actually wins it he will say "Nadine tried to lose this for us, but we won, I am the Prime Minister"
I don't think all of those provide a barometer for this and I am based in the Chilterns area anyways.
Labour during the Michael Foot years were hopeless and the Beaconsfield by-election (where a certain grinning cottager stood for Labour) was when the Falklands War was reaching its conclusion and Maggie Thatcher's approval and support and government approval did go up. The current government has nothing of the sort to cling on to now to claim popularity.