Phil,
although you are to be commened for trying to upgrade your isulation, your terminology is a bit off.
Whilst kingspans latest booklet is a good starting point, it should be noted that insulation is only 1 of about 5-6 different factors that significanlty affect your BER rating, so asking how to achieve an A2 rating is a bit disingenuous, theres so many other factors to be considered.
Believe it or not you cant actually get a BER rating on a renovation. You can only get a BER on a new build or, from next january, on an existing build at point of sale or rent.
My best advice is to hire an architectural technician thats also a BER assessor.