Wont really make much difference. HSBC, BT, Tesco, Debenhams, Next, Glaxo, Kingfisher, Avon amongst many already here but doesnt mean any savings for Brits.
I hear they might have a small discounted rate for Vodafone to Vodafone subscribers but that will be a long way in the future and also the buyout of Telsim is still subject to approval from the SDIF Board and Turkish regulatory, legal and competition authorities.