I probably pop into an Iceland once a month when the freezer is running low to give it a top up. Most of the big brands frozen things they sell (e.g. Birds Eye, Youngs) are typically slightly more expensive than other supermarkets when not on offer, but you'll regularly find them doing a buy three for the price of two, or buy 4 for £10 etc. that will bring the prices down to less and those are what I always aim to pick up.
Someone earlier mentioned their Ridiculously Crispy chips - that's not just a description, that's what they are branded as, and they are very good (especially the skin-on variety). They also offer brands that other shops don't, or ones where they offer a much wider range than supermarkets. For example I bought a cheese and bacon potato hash last month that was branded as 'Cathedral City' that I haven't seen elsewhere and that was pretty good.