Travelling by plane with an ID

You can go to the nearest British Embassy and get an Emergency Passport, which will let you travel to Germany and back to Belgium. I also recommend that you don't let your passport run out, so I would get it renewed straight away.

10 year/full passports are done from the UK now, you apply online.

Thanks a lot for the information. How much does an emergency passport cost?
So the ID/resident card won't be accepted!

As a foreigner here, residence card is only your visa, your identification document is the passport. Eventhough chances are rare that passport is asked (especially if you online checkin and do not encounter check-in staff) for intra-schengen zone travel, it is always advisable to carry your passport with you when you travel outside Belgium.

It costs £100 (or the equivalent in local currency currently €111) to apply for an emergency travel document. The fee isn't refundable.

You can apply for one HERE

For a 10 year passport details can be found HERE

Thanks a lot everybody.

And I suppose I am unlikely to receive a passport by the end of the month!

Its very touch and go to get a new passport sent over from the UK in such a short time. All renewed passports sent overseas are done from the Liverpool office, so it all depends on how busy they are. You could be very lucky and get it back in time. I would order one now and hope you get it in time then you would not need an Emergency Passport. If it doesn't come in time then, of course, you would need to make an appointment at the Embassy.

I would contact the Embassy in Brussels and ask for advise HERE