I haven't looked at the others but the BHX-ALC is showing as a 175. Perhaps they think it will be more successful if they have less seats to worry about filling.
Given business travel seems to fall in summer, BE have to find something to do with their fleet (e.g. DUS is usually only 2x daily in summer vs 3-4x in peak). MON already want >£250 pp on BHX to Alicante, and maybe BE think they can put a decent load factor into a 175 at that price, then the fiscals make sense. But outside of school hols may be a struggle.
And BHX-Cologne? Once a day, off peak?
I'm not sure Duesseldorf could take another flight, but I agree, it's hard to see this being really appealing to business passengers. And as soon as it looks like working, surely 4U will pick it up and demolish BE's business case?
No sign of Munich where BE have applied. OK there is LH competition, but the people I talk to give me the impression that BE could make progress if they can offer an attractive late afternoon/early evening returns to the UK, as with the current flight timings, business trips can be properly shattering.