what part of the UK will you be in? note that some areas (i.e. those close to london) make the travel considerably easier. otherwise, you may find the worst part of your journey the unbelievably long time it takes to get from one part of the country to the airport (and vice versa).
it's not hard, but it is fatiguing (sp?), even if you can combine all your interviews. you just get tired of putting on a face after awhile. agreed with baseball though, it is easier if you are a strong applicant. the prices for flying from london to the USA isn't even that bad during interview season. you'll find it's competitive or even cheaper than domestic flights since it's not during tourist season.
i had very few secondaries that weren't online. i can only remember two right now (cornell and nyu). i suggest trying to get your hands on these ahead of time or making sure you have a trusted contact person at home forwarding these to you. non-priority mail takes anywhere from 4 days to a full week each way transatlantic. fax is very helpful, but can be expensive. you may also try emailing with PDFs...