In article , "mo" nospam says...
B and Q? Or will I have to look more local?

All the floorboards in my room are different sizes - I guess I will have to
give them exact measurements? (width, height and thickness?) and they can
cut them to order?

You are joking? There are specialists who will make floorboards to
match old ones, but not usually on a board-by-board basis.

Is there a cheap material you can buy to put on top of floorboards to make
the floor totally flat?

Plywood. But if the floor is that irregular it might be better to
replace the whole lot.