I can assume you are a SPR yourself, completed 3 years.
It would be a good time to begin your search 1-2 weeks before your wife reaches the 3 years in July.
The reasons are that you can only sign the option upon your wife reaching the 3 years. As I have assisted SPR/SPR couples, usually their selection of a flat is faster in view of their past experiences in selecting a rented flat. Any earlier searches will likely be a wasted time for you because you cannot buy. In any case, it probably will be helpful to do desk research at this point in time.
Should you need further assistance then, you may write to email@example.com.