A potentially important point to make: when Mr Barker states "One tip though – please personalise your letter in some way, even if it’s just one extra line that you add." it is not a tip. From my understanding it is essential. If a multitude of identical letter's are received, 10, 1000 or even one suspects 100,000, they are treated as 1 petition. If they differ enough (one extra but unique line is all it takes) they are treated as separate and discrete petitions.
But then again, if you aren't interested enough in the topic to make the letter your own somehow, perhaps your letter shouldn't count

RH.