I agree to your understanding that if you backdate, then output tax will be charged on the sales and note payments received are assumed to include VAT and hence 1/6 of receipts is accounted as output VAT. An alternative to recover this output tax from our clients is to re-issue the invoices or issue VAT only invoices (some clients are o with this if they can recover VAT ).
You can indeed get an earlier date of registration (up to 4 years from now) if you can convince HRMC. This means you can claim upto 8 years of input VAT on goods ( 4 years registration date backdated plus the usual 4 years claim on goods) and for services - 4 years plus 6 months using the same rationale. The provisions for back-dating effective date of registration are contained in the VAT Act 1994, Schedule 1, paragraph 9(a), and this entitles a person who is not, and is not required to be VAT registered to register for VAT if he can satisfy HMRC that he is in business and is making taxable supplies.