The statement that cats cannot commit trespass is technically true, but is also a misreading of ss.4 and 4A of the Animals Act. Those sections replace a common-law doctrine called "cattle trespass", which was a specific tort (a civil wrong) concerning what happens when my cattle stray onto your land. Both old and new rules apply only to livestock, so not just cows but other kinds of farm animals as well - but not to dogs or cats. Horses are a special case (s.4A). Since cats are not livestock, they cannot be the subject of cattle trespass and are not covered by section 4.
Further, since they are animals, they cannot commit the tort of trespass at all. Animals themselves are not liable at law. As the keeper of cows, it would be me who was liable in a case of cattle trespass, not the cow. The point of s.4, and its predecessor in common law, is to make me liable for damage done to your property by my cows, regardless of whether I was negligent in the way I looked after them (it is a "strict liability"). The mere fact that they've strayed onto your land and done damage is enough.
Because cats are not cows, this particular rule does not apply. That does not mean that cats can't be the subject of other torts. In particular, cats could be involved in the tort of nuisance, which covers all sorts of potential scenarios - noise, smells, etc. This has typically happened when there are lots and lots of cats. And under the Animals Act, the keeper of any animal may be liable for damage that it does, including physical injury or damage to property.
Section 2 of the Act establishes a statutory distinction between whether I am liable for the damage, or strictly liable. In a scenario where I am strictly liable, you do not have to prove that I was negligent in my management of the animal. Broadly, that applies if the animal belongs to a dangerous species, or is unusually dangerous in itself. (There has been very complex litigation on this point which I am not discussing here.) If my cat is not a real vicious bastard of a cat, but just a regular cat, then I can still be liable for damage that he does: it's just not automatic.
Following Rachael Mulheron's Principles of Tort Law (CUP, 2nd ed., 2016), for an action in negligence there are several key points -
Proving that D, the owner/keeper of an animal, owed C a duty of care, where C was injured by that animal, has been a straightforward task.
The court must be satisfied that a reasonable person in the position of D, the keeper of the animal, would foresee a real risk of injury to C arising from D's particular acts or omissions in dealing with the animal. Otherwise, in the absence of that foreseeability, a reasonable D would have done nothing different in response to the risk posed by his animal.
To fall below the standard of reasonable care as a keeper of an animal, D must have failed to do that which a reasonable keeper would have done (to supervise/fence/control, etc, the animal), or have done something which a reasonable keeper would not have done.
In damage-by-animals cases, C must prove that, as in the usual negligence action, D's failure to supervise/handle/care for the animals caused C's injury on the balance of probabilities.
As in other negligence cases, there are various defences; if you hassled my cat and it scratched you, then that's your fault and not mine.
From the material quoted above, we can see that for a normal cat in the UK, where the owner is not doing anything unusual by letting it roam freely outdoors (as is typical for UK cats), it is going to be hard to meet the tests. Things may be different if D knows that C is allergic to cats, or that the cat has been eyeing up C's delicious prize goldfish, or has a bone to pick with D's own cat and is likely to attack her, or something. Most of the big cases under the Animals Act have involved animals who are more likely to cause damage, such as dogs, horses and cows; a cat is only legally different because (1) unlike for dogs, horses and cows, there are no special rules applying to cats, and (2) factually, most cats don't do much harm.
To that point, even if D is liable in the way described, C may not get very much out of them by way of damages. Defecation in C's garden is not pleasant, but is probably not worth a lot of money either.