If you call the RSPCA they will not immediately pick it up, they would wait to see if the family returned to collect it. Also if they take it in and it is not found a new home in s few weeks they would put it to sleep.
I would suggest the Cats Protection, or if you look on here -
Cats for Adoption from Cat Rescue Centres across the UK, on Cat Chat - you can search your ares for cat rescue organisations and charities.
What general area are you in, I am in Devon and with Cat Action Trust and we have a foster place available here if needed.