Can you put 205-55 R15 tires on Astra SRI instead of 205-55 R16 tires?

Of course you must know you're going to have to change wheels also as you cannot mount a 15" tire on a 16" wheel. So if you're going to buy a new 15" wheels then you need to mount 225/55-15 tires on them to keep everything as close to the factory specs as possible. This will be an Excellent swap.