Original Wooden Floor

Hello

We stripped down our original kitchen wooden floor a couple of years ago and
we used a poor weak matt finish. It now looks tired, so was looking into
spending a day or two on it.

It is a old floor and there are gaps between the boards, under the floor is
a basement room and one of the problems is that the room is freezing in the
winter.

What can I use to fill the gaps in, I suppose I could insulate the gaps
between the joists from underneath, but all dirt and debris will still fall
between the boards.

After I have filled in and sandered I will use a gloss yacht varnish me
thinks??

Any advice on this please??