The deposit scheme in Germany is a huge cash cow as people pay for the deposit but then never return the container or the container gets destroyed, label comes off and gets rejected by the machine, etc.
And as you said, people leave empty bottles around public bins for homeless people to collect. However, this slowly became an accepted method of “income” for them and you see them checking every rubbish bin for empty bottles. (If a bottle isn’t quite empty, they’ll empty it onto the pavement.) And there are even territorial fights.
Also, lots of fraud with fake deposit coupons (you deposit the bottles in a machine, machine prints a coupon and you take that coupon to the till where you get your money - people now find someone with a label printer and print fake coupons to cash in).
And I loathe having to carry an empty plastic bottle around all day when I’m not near any place to return it. You can’t even squish it as then the machine won’t accept it. Which also means you’re taking huge bags of air to the store every few weeks.
I don’t see any issues with empty bottles and cans around London. Definitely not more than in Berlin. So I can only assume this scheme was proposed so that a few people can fill their pockets with the expected money. As always.