I am going to snipe a bid from the US on ebayUK and am wondering how many seconds to put in as my lead time (I want to consider the extra time it may take to get from the auctionsniper server to ebayUK if necessary).

One lead time is as good as another. It’s more of a personal preference. AS will adjust the lead time based on ebay’s current response time to avoid not-it-times, which there hasn’t been any reported on this forum for a long time. Some forum member might tell you to use some number that they think is magical, but again, that’s just their personal bias.

Here’s what the FAQs have to say about it:
http://www.auctionsniper.com/faqbasics.aspx#basics13