Plums and cherries flower ahead of apples. Peach is even earlier but probably not appropriate if you're adjacent to moorland! In this situation, do think about damson, bullace and cherry-plum (a little round plum, not a cross between the two!) for early flowers. They are all that bit tougher than the larger cultivars and make very tasty preserves. Likewise crab apple. All these are very typical of trees of hedgerows bordering moorland (depending on how high and bleak your moorland actually is!). In Romania I saw them growing as specimens on the high commons, where shepherds take the livestock up from the towns every morning and back again at night.
My personal favourite supplier of fruit trees is the Agroforestry Research Trust in Devon. Close seconds are Deacon's Nursery on the isle of wight and Keepers nursery in Kent. All do mail order and have helpful websites to help you choose varieties.
Finally, don't forget soft fruits. Currants of all colours are a traditional first source of nectar and a good way of getting your fruit season off to an early start. Raspberries, blackberries and hybrid cane-fruit come later and can be an important source nectar.