My advice would be go for a brand that has not altered their lensmount
significantly as it has been updated.
My simple reason is that you might decide to go digital and if you've
bought lenses, you may as well be able re-use the glass you've got
rather than spending even more money.
That (unfortunatley) rules out both Canon and Minolta film SLR cameras,
because both companies essentialy left users of their manual focus
camera users high and dry when they implemented autofocus in their
cameras (Canon went from FD to EOS, Minolta went from MD/MC to Dynax).
Pentax and Nikon would both be good choices, as they both have a vast
array of lenses that that could be used and (some) bought (relatively)
cheaply.
Also, both brands have kept their lensmount relatively constant and have
(pretty much) only added electrical contacts to implement AF and
electronic aperture control for newer lenses.

My personal choice would be a Pentax K-1000, as they are relatively
cheap (much cheaper than your budget of 300 pounds) and the lenses can
also be used on the latest Pentax DSLR camera.

Practica, Pentax, Nikon, Canon, Chinon.  A fully manual camera will teach
you more, and more quickly. It might be better to find a good second-hand
camera dealer and listen to his advice, rather than go out looking for a
particular model at the 'best' price.  There are some excellent lenses and
accessories for the 'old fashioned' 42mm screw mount, or in 'T-mount'
adaptors with basic couplings.  My first 'serious' camera was a Zenit 35mm
SLR with no slow speeds, pre-set aperture, and a mirror worked by a thread,
but it taught me a lot.  I doubt if many of them have survived - they were
like tanks on the outside, but very rickety inside.  Soviet era Praktica
SLRs were very good value when new, and many are still serviceable and
affordable - some showing little sign of usage.

I'm a rangefinder fan, I find them easier and quicker to use than SLRs for
most things, and there are some good ones being made again - with
interchangeable lenses (Leica M or screw mount, mostly) and manual
controls.  I don't think you'd get a wide choice of Leica bodies within
your budget - although even 70-year-old examples are still as usable as
they ever were.  The Soviet 35mm rangefinder cameras were never easy to use
or reliable (Fed, Zorki, Kiev) although some of the lenses were quite good.

Used Leicaflex SL.

Why not go for a used rebel, has full manual mode, is dirt cheap, and the
lenses could be used on a canon dslr when/if you decide to upgrade.