Try Harvey Malcolm's of Liverpool. They cater to the Light Div. etc & I got mine within 14 days for £135, delivered. They do Winchester etc , so were approved when I got mine. You'll need to contact them, they'll give you a list of measurements they need & then all you need to do is wait.
Take it back to the shop after the parade and claim a full refund.
Don't take no for an answer. Rant and rave and spray the store manager with saliva. Behave in an increasingly irrational and intimidating mannor and keep turning to speak to an invisible companion. Howl like a dog during moments of particular frustration.
They will give you a full cash refund and (politely) usher you from the shop.
You will probably have to select a different store next year.
Don't buy one. I go to the parade every year and there are always old boys who can't march at the last minute because they feel ill. Just jump in as a concerned first aider, loosen the clothing, ie jacket and tie and slip away into the crowd ... might even get lucky and get one with the right badge!
... I'll get my blazer and leave now ...
But seriously ... always cold on the day and a coat is more useful than a blazer. I got a good overcoat from 'Suits You' in an outlet centre for half price ... and they do blazers.
There is often an advert in the Daily Mail for a tailor from Hong Kong -looks good value and quality.
I know you don't believe anything printed in the Mail, but the ad looks genuine enough. If you ask your newsagent you could even get yesterday's for nothing. You don't have to read the whole paper the ad is normally near the back!