I'd use 9mm mdf. Plywood has a tendency to warp unless its 18mm or so. And it's generally more expensive. 3mm hardboard has always stayed rigid for me. If you're in America and have difficulty getting mdf use chipboard.

When I made my board I bought flock on a roll and pasted it down with wallpaper paste them smoothed it our with a seam roller.

There are a lot of ways to answer your question. My suggestions are going to is going to depend on How you answer mine. Where are you, where are you going to put the board if it is not on a dedicated gaming table and finally a budget. You can buy every thing or make everything or something in between.

I have used roll flock and it has been around more than 50 years.
Sand is a double edged sword. It works well but If you use sand to surface a 3/4" or 20mm 4'X6' sheet of plywood you really have a giant emery board.
If you are going to move the board around as one section and have limited access to tools. I would recommend a 2" x 4' X 6' sheet of Pink foam. You wont have to frame it and you can edge it with the decorative duck tape that my God Children and Grand Nieces like so much. The sad thing is the Pink foam is hard to find in some localities.

I am int he UK, I' m plaaning on storing the board in a garage or shed. I haven't really got a buget at the moment, but I would prefer to keep costs to a minimum if things can be done cheaper if I DIY it.

I won't be able to move the boad around as 1, it will need to be in 2 parts so i can move it.

Is sand not recomended then? I was planning on just spraying it brown then flocking it.

Also, how is it best to stick down flock/sand? Last time I tried my board ended up really warped. I was considering hardboard because I could screw it to a strong frame.