I'm thinking 2 birds one stone here.....

I've been told I need better underfloor ventilation (there's no
ventilation at the back of the house, just the front).

We are doing some refurbishments so it's as good a time as any to rip
the floor up and put a couple of snorkel vents in the back wall.

And i'm blocking up a fireplace and apparently supposed to vent the
chimney. As it goes against the grain to send hard earned warm air up
the chimney I thought of venting the space under the floor into the
unused chimney instead.

It seems like a no brainer to me.