I hold an Egyptian passport and am traveling from Cairo to London via Turkish Airlines. I will have a long layover more than 12 hours. Do I need a transit visa? or shall I enter Turkey getting a transit visa on arrival at the airport. I have a valid visa to England
A search in Timatic gives the following result:
Transit - Turkey (TR)
TWOV (Transit Without Visa): Passengers with a confirmed onward ticket for a flight to a third country within 24 hours. They must stay in the international transit area of the airport and have documents required for the next destination.
So you don't need a transit visa. (I am assuming that you will indeed be staying in the international transit area in Istanbul, i. e. both flights are on the same booking.)