Hello to all those who have responded up to now...
As suggested, I am consulting (for the first time it seems to me) FreeBMD, WiltshireBMD... and have found mention of the marriage of Walter Mead and Ann Chapman in the 4th quarter of 1850 at Devizes, but not the birth of Samuel Mead(e) in 1853. (But why was Tisbury suggested?)
As far as Devon is concerned, I wonder if the person providing the "place of birth" on the death funeral record you have been specific enough to mention a town plus a county plus "England"... (but this is speculation!)
I do not know more about Samuel that what I already mentionned: since he was 17 when he migrated, perhaps he came with relatives, but I have not found traces... unless...
Thanks for all your contributions!