I must admit I'd go to Waitrose or Sainsbury's deli counter and get them to slice (very thinly) an amount that you think is sufficient, any leftovers can go in the freezer.
You won't have the worry of cooking and possibly getting it wrong, and with the cost of buying and cooking, you'll probably work out even.
Get some really good pickles, eg Lime Pickle, Branston, a variety of mustards, especially Dijon and grain mustard, some excellent bread, again from a good supermarket or if you're lucky, a good local bakery, and, with lots of lettuce, cucumbers and cherry tomatoes, and you'll have some great sandwiches.