Both releasing them and leaving them are illegal under the Prevention of Damage by Pests Act 1949. You must kill them if you trap them and you are legally obliged not to let them live in/on your property.
Are you able to source this in relation to Prevention of Damage by Pests Act 1949 where relevant to OP, outside of local authority requirement or where "substantial" numbers are mentioned? We're not talking about a thousand sewer rats.
You're supposed to kill all pests. Because by definition they're pests. This applies to mice as well. While you might not have an issue with them in your garage/barn eventually they'll knaw their way through something important and you'll wish you'd been a bit more proactive.
This is partly subjective and also relative case by case. There are ways to be "proactive" to protect from damage in the first place too e.g. by means of using correct cabling, securing building etc. We've not had any evidence of damage in our garage the 5 years we've been here, they're free to use it. It's too much of a risk to do anything about it due to the range of wildlife we have, from hedgehogs to stoats. Our single storey extension loft is another matter, we trap them where required.
To see them as a pest straight off the bat without causing an issue or attempting to prevent one is sad.
They're not cute, they're not cuddly and they do injure (even kill) people with the germs they carry.
Subjective, mice and rats can be (and are by many) seen to be cute (people keep them as pets you know....it's not a reach to consider them in the wild as "cute") and an important part of our natural ecosystem. They are other animal's food for one and we are morally obliged to ensure ecosystems and all life is sustained. Sure, sometimes there is a need to control species, but we can and should do this as natural and humanely as possible.
As for germs, barely even worth mentioning unless a cause for concern with agriculture, food production or concerned about a new Great Plague or Black Death scenario...
Ultimately, protect fixings and prevent buildings from a potential "pest" in the first place and should you need to kill, do so quickly and humanely with a snap-back trap (there's multiple types and different way to use the bait) or gun. Otherwise, you're pretty much sub-human yourself, IMO.