I think it depends very much on your circumstances - financial, where you live and how you intend the cat to live.
If you have a good amount of money put aside and can afford to pay out what could be up to a £10k bill, then you probably don't need to worry.
If you intend for the cat to be indoor only, there is much less chance of major injury or catching anything. That said, Boycat developed CKD and would have cost me about 5 years worth of premiums in the 4-5 months between diagnosis & him being PTS if he hadn't been insured. This at a time I was out of work and couldn't have afforded it really. And Toddlerteaplease's indoor Persians cost her insurance company around £24k over a few years!
If you live in a busy built up area, an area where cars/lorries charge through at high speed, or a "rough" area where there can be unpleasant teens roaming around, then I would definitely get insurance as these all lead to higher chance of injury.
If you do insure, get lifetime insurance. DSis didn't have insurance for one of hers who developed epilepsy, and he was costing her much more per month in meds than the insurance premiums would have in the first place. And as this went on for a few years, Lifetime would have covered it as an ongoing condition whereas without Lifetime they will stop paying after the first year an illness is contracted.
As Beamur says above, do look into a vet scheme if yours does one. No it won't cover any accident/injury/illness, but if your cat is going to go out then monthly flea treatment/worming can add up. As mine are indoor so only get flea/worm treatment a couple of times a year, they just have one annual visit for jabs and MoT and it's only just cheaper for me not to have the plan.
Finally, please pay the cat tax. Photos of recently acquired cats are mandatory on The Litter Tray! 😁