Where being VAT will make the big difference is if you are wanting to sell to VAT registered businesses, who by definition will probably be larger customers.
Looking at the buy for £10+VAT/Sell for £20 inc VAT analogy again, the unit price to a VAT registered business with you not VAT reg will be £20, and they can reclaim none of that.
If you are VAT registered, they can reclaim the VAT element of the £20, so the cost to them is £16.67
Thus you can see if they are buying tens, hundreds or even thousands, the advantage to them to deal with a VAT registered business can become considerable.
You therefore must look at your intended customer base, and decide if VAT registration and the additional admin that it brings will be a benefit to you in terms of your target customers.
You don't have to register immediately, and when you do decide to register, I believe you can still backdate a reclaim for VAT on some inputs for a few months before - but an accountant will advise better on that.