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Hi. My name is Tony. I'm looking at your question now and will post my answer or ask for more information here in a short while.
You should qualify for entrepreneurs' relief on that part of the gain from the sale of the business which will limit your CGT rate to 10%. See HS275 here. ER cannot be claimed on the gain on the flat.
Assuming the property was jointly owned, you cannot divide the sale proceeds in the way you describe so that you can claim your brother's deferral allowance. In any event, you cannot defer a gain by investing in a buy to let property. You can only defer a gain by investing in business assets and buy to let isn't classed as a business asset I'm afraid. A furnished holiday let may qualify. See HS290 here.
I hope this helps but let me know if you have any further questions.
You can try and get ER on the whole property but I don't think HMRC will accept such a claim on the flat regardless of the whole property being classed as commercial.