Tight.
30 minutes Terminal B immigration
15 minutes additional Terminal B bag reclaim and customs
10 minutes monorail to Terminal C
10 minutes security at Terminal C
10 minutes to get to your gate.
However it is a regional jet flight so boards fairly late. You could make it.

a) if you travel without luggage you'll save the waiting-for-bag-at-Customs bit
b) if you OLCI 24 hours before your flight, you save time at Terminal C
c) if you're elite on BA, you may get your bag sooner at Terminal B
d) if you're elite on CO, you may get through security faster.

Alas this is the last flight of the day so if you miss it you're stuck overnight. Unless you can get them to rebook you to Hiltonhead, Columbia or other airport an hour or more from Charleston.