I have no experience in Iceland, so I am speaking more generally.

It strikes me that bringing your food from UK would require you to take all of it with you everywhere, starting out your trip with 9 or 10 days worth of food (and food weight). If you will be staying in hostels mid-trip and the hostels are located in towns, you may be able to eat fresher food, or possibly carry less weight, by buying your food on the spot. Or you could mix the two strategies and bring only some of your food and buy the rest on the spot. The fewer days of food you must carry at a time, the better, in my opinion.

As for gear, the easiest way to make your pack too heavy is to carry too many clothes and in my experience beginners invariably take too much clothing. Also, avoid cotton. Synthetic fabrics can be washed by hand and air dried in a reasonable amount of time. Cotton takes forever to dry.

NB: I would love to do what you are doing.