BRS-BRU
Average loads throughout the year would certainly be in that area; probably a bit higher because the route traditionally drops one daily rotation in the main summer holiday period.
It's a route of over 20 years' continuous standing, originally by Sabena then by SN Brussels/Brussels Airlines, although there may have been a short hiatus at the time Sabena disappeared.
A decade or so ago it was operated at 4 x daily on weekdays by SN's ARJs/Bae 146s and at one time carried over 100,000 passengers per year.
In recent years the route has been operated for Brussels Airlines by bmi regional, then by Flybe. NCL-BRU was similarly operated but SN pulled that route last March even though it was averaging slightly higher loads than BRS-BRU. I wouldn't have been surprised to see BRS-BRU go the same way, despite its substantial passenger number increases in recent years, so perhaps the yield is better than was the case on NCL-BRU.
BRU is not a huge destination from the UK. A look at the most recent CAA stats shows scheduled routes to BRU from only six UK airports, viz: LHR, MAN, EDI, BHX, BRS and EMA, although bmi regional is to commence NCL-BRU this spring on its own initiative.
As you point out a 50-seater would be more than adequate most of the time but there would have been times last year when some flights carried more than 50 passengers, so the smaller bmi regional aircraft may put a slight brake on continued passenger growth.