No hanging decorations

Are there any beams? It's probably a sellotape/blue tac rule but if there are beams then you can put bunting up without breaking the rules.

Other things are large structures or shrub trees with lights in them 2 at the door either side and then others about the place. The light up letters would also work

How about some uplighting around the room? It can look really nice and as the uplighters are freestanding it wouldn't break any rules.

You could hire one of those twinkly backdrops? They stand up on their own, or get a load of battery candles and dot them around the room and on tables?

We had a village hall and had the same brief which we were only told 4 moths befofe, my sister had already sewn 180 meters of bunting so I wasn't impressed, luckily when we went to decorate the day before my MOH's BF spotted a few old nails on the walls at the top so he hung the bunting from them and it looked perfect.

Maybe go and look, you may get lucky.

Linking balloons in an arch shape the middle sweet of balloons filled with helium to keep the arch shape.

Do you know anyone very good at diy that could make a couple of tall thin stabs you could place in the corners and connect bunting to them.

Helium balloons with long weighted ribbon on the tables so the decoration goes up towards the ceiling and not down from.

Could you connect string or ribbon window handles to window handles (if they're high enough) and tape decisions, banners etc to the string/ribbon

Can you use window stickers you just wet to stay in place.

Mosaic jars with lights in to project colors onto ceiling and walls.

Hope anything on this list helps