I agree that, ideally heat resistant cable should be used. But the temperatures near the terminals of most boilers are not that high. So, provided the cable does not touch the heat exchanger or any hot pipe, it should be OK.
I would always use heat resistant cable for the motorized valve etc as they often come into contact with the pipework.
Not wired many boilers then , Yes the terminal strip is in a cool area but you tend to find there is hot pipes and hot bits of metal the cable has to pass before it gets there.
Why would you use heat resistant cables on motorised valves??? They already come cabled up with heat resistant cable/flex. Never seen one come that you have to wire from scratch.
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