Combinatorial Approach
Yes, I agree with you Muppet06.
Time to brush off the old combinatorics skills .....!
The number of distinct draws is (assuming that 2 draws are the same if the order of pairings and order within a pairing are unimportant):
8!/(4!*(2^4)) = 105
The number of distinct draws where no 2 English teams meet is:
4! = 24
The number of distinct draws where all 4 English teams meet is:
3*3 = 9
Thus, the number of distinct draws where exactly 2 English teams meet is:
105 - 24 - 9 = 72
So, the probability of a given draw containing one, and one only, English team pairing is:
72/105
Thus, given any draw the probability of a particular pairing being an all English tie is:
(24/105)*0 + (9/105)*(1/2) + (72/105)*(1/4)
(I think! I am more than happy to be corrected)