Dashing Tweeds can help here, they have some OTP items. And if you can find a jacket that fits you could try the other country wear shops around Jermyn St and Piccadilly such as Cordings. But note that a lot of country wear is expected to be worked in, and generally comes up larger and less fitted than the hunting wear.
You can still find that, just not on Saville Row.
Dashing Tweeds can help here, they have some OTP items. And if you can find a jacket that fits you could try the other country wear shops around Jermyn St and Piccadilly such as Cordings. But note that a lot of country wear is expected to be worked in, and generally comes up larger and less fitted than the hunting wear.