You don't just take it home as a pet!!!
My lovely tortoise escaped from my garden just over a week ago and thanks to lazydog and the power of t'internet, we've tracked him down to where he was last seen.
A woman had him overnight in her garden, put there by a dog walker. He then collected him the next day as a pet for his grandchildren. 
Now, is it just me, or would you not make some attempt to find the owner? We have posters up and leafleted and I do understand that he may not have seen these, but really? Tortoises are not native to the UK; they certainly do not turn up in fields; they are not wild animals that can be taken from a field as a pet!!!
Surely ianbu to think you might phone the vet/RSPCA/police if you found a foot long tortoise while walking your dog?