I think the bad news for you is that this plant is really a Mediterranean one, and will not survive the cold frosts in this country. (My two didn't anyway). If you're still determined to have a go, try planting it in a large container with John Innes No. 2 in a sunny spot and in the winter move the container to the most sheltered spot in the garden, preferably to a greenhouse. If you haven't got a greenhouse you will probably need to cover it round with sacking during the winter. I think this plant will not fruit until the tree is several years old and am not sure whether you need more than one of them so that they can fertilise each other. Googling "Growing avocados" might give you more information. Hope it doesn't die on you after all the loving care you've given it to date.