There’s been just one by-election in past 16 years caused by death/illness of CON incumbent
There is a lot of hope on the Labour side that the small CON working majority with the DUP could be cut as result of a by-election loss.
The only problem with this is that historically there have been very few by-elections caused by the illness or death of the CON incumbent. Since 2001 there has been just one compare with 15 Labour health/death ones. Note that I have excluded Jo Cox.
Most by elections in Tory seats have been caused by the voluntary resignation of the Sitting MP such as David Cameron in 2016 or the weird resignation by David Davis in the 2005-2010 Parliament when he quit and stood again to make a point the purpose of which has long since been forgotten.
In 2014, of course, we had two resignations in CON seats caused by the defections of Douglas Carswell and Mark Reckless to UKIP.
Last year, Zac Goldsmith, quit his Richmond Park seat seat and the party to fight a by-election over LHR3. He regained it on June 8th by 45 votes a margin somewhat down on the 23k of GE2015.
In the current parliamentary situation the TMay’s Tories are going to do everything in their power to stop voluntarily by elections.
So we could experience something like the 2001 to 2005 Parliament when the Tories did not have to defend a single seat.