If you wanted to rely on just the National Rail Conditions of Carriage and the Routeing Guide, then I don't think you'd be able to find anything stating what a Travelcard is valid for, so it seems you couldn't justify using it on anything other than a return journey between the origin and London!
In order to justify use of the Travelcard element within London, you can turn to the
TfL Conditions of Carriage, which states that a Day Travelcard can be used for "Unlimited journeys on Tube, DLR, London Overground, Elizabeth line and National Rail services within the zones shown on the ticket. [...]" but in the very next paragraph you find out that "Off-Peak tickets can be used from 0930 on Mondays to Fridays".
However, if you instead turn to the
National Rail Enquiries website, then you will find a statement about unlimited travel on various services, followed by, under "Where and When Can I Use My Travelcard?",
The statement is ambiguous as to whether "valid for travel" means valid just for the journey towards London or valid for the Travelcard element within London, so resolving the ambiguity in favour of the customer results in the conclusion that the Off-Peak Day Travelcard from Coventry is valid for use in London from 05:40 or 07:03 on Fridays (depending on whether you count the departure or arrival time of the first Off-Peak train!).
Bonus challenge: Talk your way past gatelines and ticket inspectors in morning-peak London with an Off-Peak Day Travelcard from Iver, bought with an itinerary Iver 00:16 to Paddington 01:13. This costs £15.20 so will save you 25% compared with a standard (inboudary Z1-6) Anytime Day Travelcard for £20.30! (But alas it's still more expensive than the Oyster Z1-6 day cap at £14.10...)