|Nicey replies: Neil,
First may I take a moment to compliment you on your forward planning and foresight. If only more people would seriously consider what biscuits they were going to eat in a foreign country some 3 to 4 months before visiting it then I'm sure their trips would be that much more successful as a result.
As you are going to Scotland I would think it none too bad to focus on the local stuff. Absolutely anything by Tunnocks will prove a source a great learning. Simmers Abernethy biscuits are also very Scottish and well worth investigating. You'll probably quickly tire of the all too predictable shortbread which will follow you everywhere you go, until probably in mild desperation you'll succumb and by some in duty free on the way home. Other than that I would say try the Penguin which inspired your own Tim Tam, this could be seen as a very academic exercise, so I suggest you work up a good appetite and pop away a couple of them before coming to any conclusions.
As for Marmalade, there is much to choose from as you say. Steer clear of all those jars of "With Whisky" nonsense that will be keeping the tins of shortbread company stalking you around Scotland. Much better just to drink the whisky and eat the marmalade toast at the appropriate times. I find that there is much to be said for the small producers of preserves who make something with a bit of character. Other than that I always like a bit of Frank Coopers whilst Nanny Nicey likes Roses Lime Marmalade as do the YMOS.
Finally if you find yourself in Annan in Dumfriesshire as we did earlier this year be sure and visit Scotland's third best fish and chip shop, The Cafe Royal, where the great poet Rabbie Burns wrote the immortal, "The Deil's Awa Wi' Th' Exciseman ", but steer clear of the deep fried Haggis in batter.