Just want to double check something, so every time i make a cut of the
plastic pipe, I need to insert a pipe insert before connecting to the 90
degree angle or T joint etc?

Yes. The inserts are very cheap, but must not be forgotten.

Does this help stop the pipe from collapsing slightly?

Yes.

The thing that worries me about using copper pipes is not being able to
heat
the joint all the way around due to confined spaces under the floor
boards.

That isn't a problem. The copper conducts heat very well and you don't need
to heat behind. The main issues with soldering are making sure all metalwork
has been properly scrubbed with wirewool/abrasive paper, ensuring that there
is no water in the pipework which will prevent a good join, and ensuring you
don't set fire to anything. I always have a proper fire extinguisher with
me, such as a 6kg ABC powder, or a 2kg CO2.

However, in concealed locations, plastic pipe is far easier to work with.
You can thread it along joists and feed it into holes etc, much more easily
than copper. You need far fewer joins and don't need to make precise bends
with a pipe bender. I only use copper when on show, or when rigidity is
important, such as where pumps and valves are being held. You usually must
use copper within a certain distance of a boiler, too. This will be stated
in the manufacturer's instructions.

I tend not to use push fit much with plastic. I find compression joints to
be easier (it is easy not to push the pipe in far enough with push fit when
in tight spots) and, more importantly, they are a fraction of the price.

Christian.