"The Adoption Act 1976 provides that for the purposes of the Act provisions of the law of intestate succession applicable to the estate of a deceased person shall be treated as if contained in an instrument executed by him, while of full capacity immediately before death. Thus if an intestate dies after 31 December 1976, adopted children will be entitled to share in the estate as if they had been born in lawful wedlock". - Williams on Wills para 75.8
In english - if you are adopted, you take under the intestacy of your adopted parents. Thus if your adopted parent(s) have 2 natural children and 1 adopted child, you are each entitled to one third. You do, however, have no claim under your birth family. This would also have to be a legal adoption, rather than an "informal" one.