Shot One - I agree with you. However, as SkyDiver said - it wasn't so. Therefore to not include it for years and then add it when it looks like you might miss the 2% target might look like a fudge. As they say "you might think that, I couldn't possibly comment..."
As an aside, my understanding is that the separate accounting was due to the RN demanding that measure when they took over the deterrent role. Stopped their portion of the Defence budget being gobbled up entirely by the hugely expensive purchase of the initial SSBNs. Also allowed them to not have to include the cost for the replacements from their share of the pie - not sure how that will work now?