Seasonality also has a climatic / geographical element. Hereabouts, I perceive that some tups (Charollais, Shetland) are always ready if called upon, but most of our ladies aren't interested until the weather cools and the days shorten. (And it has to be both, IME.)
Last year we bought some Mules in from a farm which lambs later than we do. The Mules we already owned all came a-tupping before the bought-in ones. It'll be interesting to see if our new Mules come a-tupping a little earlier this year.
All of which said, farmers all over the country, including up the road from me here, produce Suffolk lambs indoors in December / January, which would suggest that the breed is amenable to tupping and being tupped in July, even in the far north of England.