While I don't disagree with licensing dogs (or perhaps the owners) I think the suggested sum of £50 is waaayyy over the top, so far that i's just not going to happen.
People will still keep dogs, responsible ones will buy a license (thats likely the same ones that do pick up the doggy poo) and the others will just not bother.
With either the license or the doggy poo.
And there is no point saying dog wardens will take the dogs away, that just wont happen to them all either, only a few.
Some people just don't give a toss, about either keeping to government rules or in keeping our streets clean no matter what suggestions or ideas you have, you cant change it because they never will.
This is NOT an argument against your suggestions, just an opinion.