check out bags and 72 hour free visa in Beijing airport

Hello,
we are flying from UK to Thailand with Air China via BJ Airport with a 18 hour layover landing at terminal T3. Because we have the long layover would we have to re check our bags? also, we hold UK passports and would like to leave the airport to site see during our layover. Would we be eligible for 72 hour free visa? and how would we get the visa? thanks for any help you can give.
Valerie