Hi , 6mm plyboard isn't expensive and I have been cutting it with a stanley knife with ease for thirty yrs. You won't find a cheaper option that gives a good look with anything else.

Regards James

You don’t need 6mm ply if the existing floor is in fairly good condition. Try 3.5mm ply or even hardboard, both of which can be cut with a knife, to keep costs down

I agree with the above comment. Marine ply is brilliant for works like this and is probably your cheapest option. If you want an underlay for vinyl, it is something called everlay, which you lay before the vinyl and spray glue it around the edges to make sure it doesn’t move under the vinyl. Although the everlay won’t get rid of any bulges that the subfloor may have.

Hi, I agree with the two above. 6mm ply is the best option on floor boards and chip board. But I would make sure you smooth your board joints in case any are un level. I use Ardex feather finish.