One of the reasons tour operators request travel insurance is that they are liable to get you treatment in the event that you fall ill or have an accident and don’t not have insurance. With this in mind I can understand why they insist to see insurance documentation.
The tour operator certainly has no right to just "add it on" but it is certainly within its rights to prevent you from travelling unless you show the documentation.
All insurance sold in the UK is usually reasonable when it comes to accident cover so I cannot see a situation where the Tour operator could claim it wasn’t sufficient
Smala01
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