At your young age, it makes sense to take some risks and pursue your dream career in animal welfare at the RSPCA, even with the pay cut. The fulfillment and passion you will get from that work is invaluable.
The cost of living is always a concern, but you may be able to find ways to supplement your income or reduce expenses if needed. Perhaps you could find a part-time job or take on freelance work to help balance the lower RSPCA salary. Or maybe you continue living with family a bit longer to save on housing costs.
Additionally, getting your foot in the door at an organization like the RSPCA can lead to better pay over time through promotions, raises, and expanded skills. The experience alone will be tremendous and may open doors down the road.
My advice would be to take the leap and accept the RSPCA role. Follow your heart. You don't want to look back with regret, wondering "what if?" This is the time in life to pursue passions, gain experience, and build a career you love. The financial piece can be managed.