No set rule to finding best price. It could be direct on their website.
August 2006 we booked via Airline Network (now netfilghts.com) at £330 each Direct with BMI was £530, Virgin was £650.
As I said coming home with BMI this August direct with BMI was £450 just for one way

booked it with Orbitz.com at $470 (£235)
We go upto 4 times a year so have to find best prices possible, when looking for flights I spend upto 2 hours every day looking, searching 3 and 4 times every day. Its very time consuming but saved a fortune compared to frightening prices I see some paying.
Never done online check in with BMI, only Virgin.
Anyone flying BMI remember you get 100% miles added to your Virgin flying club account.