It might help if those signing were actually resident in the U.K.
Also UK citizens living, or working, or on holiday abroad and absolutely entitled to sign. 'Abroad' could be anywhere in the world because, believe it or not, modern technology (mobile phones, internet and satellites) makes the UK government petition website accessible from just about everywhere.
If you look at the underlying data you'll see that most signings from 'abroad' are very low in number in individual countries. More commensurate with Brits abroad than with bots.
Anyway, why are you bothered, Leavers? I thought that you think that cheating is absolutely fine in our democracy 