Probably not that much, to be honest. Most people here in the UK neither know nor care about him. Which is a real tragedy, but people just don't understand the technology side of it, and probably don't care anyway as they think it doesn't apply to them.
These stories have been popular with the Guardian's standard readership, but inside the UK at least, I doubt it has increased its market.
The Guardian have had a large focus to try to pick up US readership for a while (e.g. They launched CIF America).
The NSA coverage has probably helped them in that mission, in fact it is probably motivating a lot of their NSA coverage, the UK "switched off" over the issue almost day one.
These stories have been popular with the Guardian's standard readership, but inside the UK at least, I doubt it has increased its market.