We have a lot of dangers in urban and suburban areas, like cars, dogs,
poisonous chemicals, and nasty people who enjoy hurting kitties. Even in
rural areas, we have plenty of things that like to *eat* kitties.
It isn't safe to let them out of your own yard here no matter where you
live. The barn that I go to recently lost two cats over the last year to
coyotes.
I do think cats are happiest when they have a yard to play in, though,
and my one cat loves to walk on a leash. I'm trying to get the other two
to want to go out in the grassy area of my condo complex with me.
They'll sit on the balcony, but get scared downstairs (cars, dogs,
strange people, etc).

Actually, in most urban and suburban areas in my state (and many others)
it is illegal to allow your pet (cat, dog, ferret, or whatever) off your
property unattended and leash laws apply to cats as well as dogs.

Try propping it up a bit, so it's open enough that just a little push
lets her out. She should be able to see that it leads outside to make
her want to go through. If she's not comfortable with it touching her
yet, prop it all the way up and gradually lower it. This is how agility
dogs are taught to do the tunnel.
Be a good neighbor, though - even in the UK, many people don't like
strange cats doing their business in their yards. Try to keep her in
your garden if you have neighbors who dislike cats.