A straight line, down from the UK, takes one to New Zealand I
believe.

As others have noted, not exactly.

One wonders about gravity at the core, :-)

Inside a sphere of uniform density, the force of gravity is proportional
to distance from the center. In other words, if you descend straight
down at a uniform speed, your weight decreases at a steady rate until
it reaches zero at the center. Then for the second half of the trip,
it increases at the same rate but, of course, with the opposite direction
as "down".

The Earth's deeper layers are somewhat denser than the rest, so that
pattern wouldn't be followed exactly, but it's close enough to the true
picture.

For an off-center tunnel, like from the UK to NZ, in the middle part of
trip the direction of "down" would rotate continuously through 180 degrees
without your weight ever decreasing all the way to zero.

Hope this helps. :-)
--
Mark Brader, Toronto "If it's on TV, it has to be true!
(I read that on the Internet.)"

My text in this article is in the public domain.