Ah okay thank you. Do you know if CIV is only available at ticket offices?
To my knowledge it's only available at ticket offices now. RailEasy used to be able to sell them but I think just sells normal tickets to London Terminals (which don't have CIV protection) now so it's a ticket office job.
In any case it seems to me a rather poor situation that the 'correct' ticket is not publicised and non trivial to obtain even if you know about it. Or, for that matter, that Eurostar has given up offering through tickets in the UK.
I think the reality is that there's no-one who cares enough to make it happen. Eurostar are quite happy filling their trains by people booking through their website and for the TOCs it's a niche market and they probably see it as a) a revenue risk as people without Eurostar bookings might chance it and b) a loss maker as the tickets are priced below what they'd sell for someone making journeys to London normally in many cases.
I thought I'd seen a case where CIV tickets were similarly priced to the Anytime and Off Peak regular tickets but not the Super Off Peak.
Perhaps, it's not a subject I've studied in any great detail as I've only had to purchase them for trips to/from Darlington. Where the 1st AP price was very competitive (£50/60 from memory with railcard, this was for trips in 2018/19) compared to the public rate 1st AP. Looking at it today the cheapest normal flexible single* is the Super-Off Peak single priced at £147.50 compared to the Euro Standard Single of £74.50.
I'm sure there may well be cases where the CIV ticket is a similar price to or perhaps more than a normal ticket to London but, again, it will depend on your risk appetite. Some people may prefer to save £20 and buy a normal ticket to London because everything will probably be fine. Personally I'm happy to spend a bit extra (though, in my experience that's never happened, it's always been cheaper) to ensure that I have a contractual entitlement to onward travel without arguing.
Also, while I believe that holiday insurance tends not to be all that useful when it comes to missing trains, I think that missing a connection to/from Eurostar at the UK end will often be covered just as missing a flight out or a connection due to a late incoming flight will be (assuming you've been reasonable about connection times etc.)
Personally I'd rather have a ticket that entitled me to onward travel rather than having to fall back on travel insurance and be out of pocket whilst that's sorting itself out
*There is cheaper Super-Off Peak Single available at £74.30 but that can only be sold as part of a return journey.