Good Morning Veebee,
Usually temperatures in Tuscany can be quite unpredictable. Historically however, the temperature in October tends to be highs of 22°C and lows of 11°C.
By 'smart casual' wear this tends to mean any of the following: chinos, jeans, blazers and jackets, converse shoes, polo shirts, blouses, skirts, trousers, cropped shorts.
With regards to your flight, you can check in your luggage early at LGW:
British Airways
Opening hours: 16:00 - 22:00
Eligible flights: All next-day flights departing before 15:15. Use the self-service check-in kiosks, then deposit your baggage at the fast bag drop in zone A.
Cost: free
Other conditions: all passengers must be present at check-in
I hope this helps.
Kind Regards,
Amy