I realise that the horse has bolted and you didn't expect the kickstarter funding to have taken off as it did but you should have taken advice from an accountant or tax adviser on the tax implications beforehand. Accountants and tax advisers fees may appear hefty but find yourself a good one and they are worth their weight in gold.
As the investors will be getting something in return, the money you raised is taxable income and is subject to VAT. You've fallen foul of the rules because your turnover won't have fallen below the threshold in the next twelve months after you initially hit the threshold and I understand your frustration at the apparent unfairness of the extra day making such a profound difference.
The HMRC staff at the level you have spoken to clearly don't understand what crowdfunding is and those at a higher level who write the letters look at the law in a blinkered fashion. This is especially true of the VAT side of HMRC.
Because of the nature of crowdfunding transactions and it is relatively novel, the law can be slow to catch up and put down specific rules (though some would argue that what existsis fine). There is a possibility of arguing when the supply was made, the basic rules for which you can read here
, and I've even read articles the idea is put forward that because there is no obligation to provide a reward to an investor according to the terms of some crowdfunding sites, it is possible to claim that any such reward that is made is a partial repayment of a loan and is, therefore, the original "investment" is not taxable income or subject to VAT. I think thats a long shot myself.
If I were in your shoes, I'd appeal and take it all the way to a tribunal simply because of the difference a day makes to your business, to possibly make the VAT people rethink and to have completley independent people look at your case. HMRC resources are stretched so take advantahge of it. If you can afford it, get some professional advice from accountants or tax advisers involved in the tax issues around this type of business funding.
I hope this helps but let me know if you have any further questions.