I had a bike nicked from the temporary bike shed about 5 years ago which was not as secure as the one old one. Bike was on a CtW scheme so was technically a company asset. Didnt bother with HR and spoke with senior Finance who handle the insurance ( not the CFO, they come & go). I claimed on my own insurance & the company’s paid the excess £250. I dealt with broker myself; all good. While people saw it was fair & reasonable to help, neither of us mentioned “liability” & they did not have to pay anything.
In the OP case I’d be very wary of pushing liability to your employers regardless of any slight chance of grounds for a claim existing. It is not a trivial thing & they should have better facilities but its not likely to bother anyone responsible in most firms too much.
A more constructive conversation might be around tax relief for capital Improvements & running costs to support secure cycle storage, changing facilities, showers etc plus all the proven benefits to the firm of having more bike commuters. CTC had info on all that, which I assume is still applicable.