Will this shrub grow outside in Northern England

Myrtus luma apiculata is just about frost hardy - it'll be the minus figures that might kill it. If you plant it somewhere sheltered, you might get away with it. you could try fleece over it if you know its going to be particularly cold in the winter.

I'm planting it between other dense shrubs about 5 feet high by about 4 feet wide and a 12 feet high tree, about 6 feet from the front of the house. Thought of putting something like a layer of gravel a few inches under the top soil to protect the roots from frost. I take it the leaves may need protection as well.

its the growing tips that in most frost tender plants suffer.some fleece will do as bamboo sais.with a lot of mine i just rap eltric tape round the leaves tite round the tips.it works on most but i wouldnt risk just leaving them open.a lot of frost tender plants actualy aclimatise after 5 years like bananas the hardy one and some ferns.