With this you could either claim through Thames Water's insurance, or you could claim through your own policy. Your own insurer would then have to re-claim from Thames Water's insurers. The advantage of claiming through your own is that it would probably be far quicker, and then let them deal with Thames' insurers who are dealing with a much bigger and more complex claim and hence it taking far longer (time in which you can't get back to running your business - or relocating to alternative premises if you can).
With business insurance, you can choose to have material damage cover (includes theft etc), and also business interruption. If you don't have business interruption cover then obviously every minute you are not in business then you are effectively losing cash. It's fairly common for businesses to go down because either they chose not to buy business interruption cover or they have underestimated the length of business interruption they need (12 months might seem like a long time but not if premises need rebuilding, planning permission needs to be sought etc - or if the premises landlord doesn't have buildings cover or drags their heels).
(I don't know whether Thames' insurance would cover business interruption or not and so am not commenting)
It can be a really messy business. Yes, sometimes insurers will try and wriggle out of things, but also, sometimes, businesses simply don't pay enough attention to what cover they are buying - or over the years the business gets bigger but they don't update the policy/inventory and so find themselves underinsured)
None of this is a comment on who should or shouldn't pay for what, just context.
