Section 14 of the Wildlife and Countryside Act ...
snip
It includes Grey Squirrel and Black Rat but not Rabbit or Brown Rat.

I don't know of any law which prohibits the capture and release of
Rabbits or Brown Rats.

One that covers vermin perhaps?

As you say that act is to preserve the indigenous habitat/species from
imports. Though the rabbit is techincally an import albeit it it has
been here a long time, the Romans introduced it I think.

Thanks for that Malcolm and Dave. AFAIK neither the brown rat, rattus
norvgicus, or the black rat, rattus rattus, are native species. The Normans
imported the rabbit in quantity. The Romans were responsible for importing
the edible dormouse, gliss gliss, that has become a major pest in some parts
of the Home Counties.

As for the OP your love of animals needs to be tempered with some common
sense. As an animal lover I deplore cruelty to most species but in the case
of vermin it is necessary to be realistic and accept that they need to be
killed for the greater good. Although rare the diseases spread by rat are
potentially lethal and very unpleasant. Would you like the death of someone
from Weils Disease on your conscience?