Yes I've taken my Morgan's to a local independent for the last 19 years for MOT but I do my own servicing etc.
It has never impacted on the resale value not having dealer stamps and I know it's all been done.
How do you know, you’ve not tried to sell your plus 8 yet 😉
I have sold a previous one for the full asking price Craig

and have had good offers for this one too and it's not even for sale so it's not a concern for me

...
I suspect this depends on the age of the likely buyer. Kiddies of say 40 or less

often put value on dealer stamps. Old farts like me put greater value on another old fart pottering around doing service in his garage. Morgan customers, by definition almost, fall into the latter category.
You try selling a Ferrari without dealer stamps for every year (and usually every 1000 miles) and you will find a big difference.
Its ridiculous I know but thats life.