I feed mine on Healthy Paws (Healthy Paws, producing delicious, 100% natural, holistic , nutritious, healthy dog food and treats.) The delivery I use is £4.99, and it only takes 3 or 4 days to arrive.
My dog does well on Healthy Paws. it seems to suit her. I look at it as one of the good healthy option dry foods at a reasonable price (about £7-something for a 2 kilo bag. Costs me about just over £30 for 8 kilos including VAT and delivery)
There are 2 recipes, Game and Millet, and Rabbit and brown rice. No crap in the food, just what seem to be wholesome ingredients.
(I always mix some home cooked meat or fish with a little vegetables in with her dry food too)

You've been given plenty of suggestions on food - you could also look at the Simpsons Premium range - their foods are grain / gluten free.

Simpsons Premium - Complete Dry Dog Food

I can imagine that carrier costs to where you are will be steep I am aware most companies have different costs for the more remote areas of Scotland.

I don't know where you live, or how densely populated it is - or even how many people have dogs - but when you settle on a food you and the dogs are happy with (most Labs are happy with anything ) - seeing if it is feasible to become a distributor for that food if it isn't available locally - not only would you be saving money, but you could keep costs down to benefit other residents (just a thought which may be completely off the mark )

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What you need to know before buying a Labrador puppy

Scottish Island = surrounded by water ?

Local fish ?

Local Fish, lol.

They are a valued commodity round these parts, not dog food.

Has anyone tried the Skinners Field and Trial Maintenance?