Spunky wrote:

We are having a flower bed around the perimeter of the garden, raised
about 32" and another in front of this one raised by 16".
There are 180 meters of wood needed if using 8"x3" for this project.
My question is, can I make these using 8x2 inch timber, and lined with
some plastic sheet to prolong the life of the timber.
The costs saving between 8x3 and 8x2 is about £300..

Would it be strong enough?
What plastic lining would you recommend?

I don't think plastic lining will extend the life of the timbers, may
even shorten it by encouraging condensation and preventing
evaporation. I used real used RR ties for a raised bed garden.
they'll last at least 20 years, most will last twice that long and
it's easy to replace those that rot. Used 10" X 12" RR ties 10' long
cost about $12 each at my local lumber yard... they are not very
pretty but they are very long lasting... they weigh about 250 pounds
each so you'll need a strong helper. Of course there are several
materials one can choose besides lumber... around here many people
build raised beds with ornamental concrete landscaping blocks... they
cost a bit more than lumber but are easy to work with and will last
longer than you.