I considered whether to have cheap plastic or cheap metal bath and
eventually settled for a cheap plastic one (probably 5mm, although I
must admit, I haven't measured it).

Although the metal ones were more solid I noticed that they tapered
considerably towards the bottom, meaning that lying in it would produce
a "penned in" sensation, so much so that even when showering and
standing up, it would difficult to lay my feet completely flat. I am
only a lanky streak of **** as well.

I assume the cheap metal ones are made this way because they are easier
to cast in this shape.

I spent ages building a very substantial wooden frame for my plastic
bath using bits of 3" x 2" and the result is a very solid cheap plastic
bath. It does stay warmer for longer too, as somebody else has already
mentioned.