Hang on, side from anything else, they said that damage is usually caused by not inserting enough seatpost. So they think that is what you did, but they want to see it still because they don’t know…
Surely if they want to see it, that means they might change their minds, make some other offer or whatever. But it certainly isn’t “NO WARRANTY!” as you put it.
It isn’t clear cut, so send it back and let them give you an actual answer because at the moment it sounds like the only one stopping a warranty claim even getting started, is you.
I think that you are wrong
Before asking me anything they told me that I could send it back to the UK but saying that there were not too many possibilities. Before asking for photos or knowing what parts I have been using! After that it was clear for me that they had not too many wishes…
So think about it, send the frame to the UK and back again almost certainly without warranty accepted… for that it’s better for me to accept the 350 pounds “offer”
Once I prove with photos that the cause was not Little insert of the seatpost (as they said firstly) now they say that the cause was the shim. Ok, does the Devinci’s owner manual avoid the use of a shim like some other brands do? NO, NO and NO.
They are not asking me to send back the frame again to see it better… the only offer is to pay for another front part of the frame