Re the ballot papers, I've read concerns from both sides. As someone who has counted elections in the past, I can assure you that there really isn't time to alter the papers like that. The boxes are sealed in the polling station, they're taken to the count, and then opened in the presence of election agents from the parties (well in this case from the campaign groups). When I did it, the first count was of the number of papers in the box, to make sure it matched number of votes recorded. As soon as the papers hit the table, people are hanging over your shoulder trying to read them. All the votes for each area were counted and verified (batches of 50 I think, although it is a while since I've done it) and only when the counting office was happy did we move to phase 2, which was sorting them out into the various candidates.
At no stage in this is there a chance to start messing with the votes. I suppose if you were really paranoid, you could assume an attempt to interfere with the entire box, but it would be very difficult to get away with that because it is unlikely the number of votes would tally. In addition, when I read the paper on how the count would be organised, it was made clear that the papers would be kept for a year. If there was a belief that entire ballot boxes were being tampered with, they could be recounted. As the ballot papers in each station have a reference number, it should be easy to spot if an entire box had been interfered with. And probably the only way that could happen would be for large scale criminal activity at the Electoral Commission, which I really think is unlikely.
On the other hand, and this is probably going to make me sound nutty, I think I might take my own pen purely so I can keep it afterwards. I'm planning on using one of my 'No Thanks' ones.