You can get an ET return flight to BRU on 29 November for £87.78. That only gets you 20 tier points but online upgrades are often available for £50 or so one way to get an extra 30.
I'd have thought quite likely there will be upgrade capacity for those flights on a Saturday, it is a risk though so if you want to guarantee it I'd pay the £60 or so for one of the options above.