I live in Brighton so a normal car service us around the £ 100 quid mark.
I did use my miles for a CDC in March with very little hesitation.
It really came down to using miles that were just sitting there or cash and I was trying to save cash. I rationalised by just adding it to the 80k miles I'd redeemed to go to BOS in upper!
The only issue is that when I called was that she suggested a 1pm pick-up which would have allowed only an hour or so in the CH!
And just in case it's not clear you can only us you miles for a CDC if you have used miles for your UC flight.