Yes I think the premium still applies, check the Morrisons' website, it's their scheme. It was 20p/kg on Shorthorn, 10ppk on all other native breeds. Father must be a registered pedigree animal and you'll need to be able to give his registered name to your buyer.
However, you can't bank on the premium of course, the scheme may have finished before your not-yet-conceived calf gets sold!
We would certainly happily choose an Angus for a first calving of a beef breed. Your vet should be able to measure the internal pelvic dimensions of your heifer if you have any worries - I don't know whether Shetlands are generally wide, or not?
The other native breed we have found produces tidy little calves that grow on well - not the fastest growers, but proper traditional beef - is the Red (North) Devon. I've had 4 Red Devon crosses from the Jerseys, and they've all been grand little calves that do well on grass.