Well I sowed mine yesterday.
It is a good question but maturity date is more influenced by the whole weather package between now and autumn. It may be a cool September! but I hope not. If it stayed above average temperature until October, a sowing on July 20th would still be possible and for you in Devon it could be worth trying.
Thanks for your comment on summer, I couldn’t agree more, after last year it feels almost bizarre, and so uplifting.