You should not be charged until connected, and if you cancel up to 14 days after connection then you don't pay any penalties. After that there's large contract exit penalties for the period of the introductory discounts, usually a year.
If you do want to cancel your order, then at the moment I'd suggest you write to the cancellation address in the Terms and Conditions. Given that VM may not be able to process the letter promptly (and their customer service is grim enough without pandemics) keep a copy and get proof of posting. Ideally you might want to send it via a service that confirms delivery, and then in the worst case if it all gets screwed up and VM try to install, or start billing you, you might have some hassle sorting it out, but your legal position will be bulletproof.
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