IME best thing to do with any complex BA Holidays is work out what you want and get a price. Then look at the MAN-LHR part separately to get an idea of the price differential between what BAH is showing you online and your actual preferred flights.
Then call them.
I usually find they are within a few pounds of my estimate. Though once they were able to access a different fare bucket somehow and it actually came out £40-50 less that I expected which was a nice surprise.