Yes I keep Golden Guernsey goats as well as British Toggenburgs. I also have started to cross breed my GG's to breed them up to British Guernsey status.
Milk from GG's does not taste any different to milk from other goat breeds, the taste is really influenced by their feed stuff and their general health and mineral balance internally. Raw goatsmilk (which you cannot sell/buy in Scotland, but of course get from your own holding) does not have the "goaty" taste that comes with the goatsmilk from the shops. Over winter my butterfat content is higher than 5% making the milk really, really creamy!
You will get about 3 -4ltrs per day during summer from an average GG, in winter that drops to around 2ltrs, and around Xmas time is probably 1.5ltrs. But I have milked my GG successfully for two years without kidding, so if you were to start off with a female kidded the same spring, milked her through and also took a pregnant goatling you would be able to keep them without a male for about two years. GG's are actually as hardy as any other breed and I wouldn't hesitate to recommend them for Shetland. GG's are really a breed for the family smallholding, where of course you want to make a bit of cheese, but not tons every week.
There was someone on this forum (but hasn't been on for a while) who I visited while on holiday in Shetland and she did have a young unregistered GG billy (but he could have had other breeds in him too, as the golden colour is no sign of them being pure), so there is history of GG's up in Shetland.
If you look at the Golden Guernsey Goat society website there is quite a bit of info. There are GG breeders further North than I am (I am in the Scottish Borders), but if you were to try and get a "mini"herd together I could assist you in sourcing stock, especially as I realise that getting goats up to Shetland is not that simple an exercise... (I have thought about getting some Shetland sheep down after I fell for a handsome tup at Cunningsburgh show - I also have pedigree Shetlands - but my "financial manager", aka OH didn't approve...)