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If you can stomach amazon I recommend this one to everyone - genuinely don't think you can beat it for the price. Welcome to borrow (SE23) if you wanted to try it out. 20m is a big room, and I think any portable bluetooth speaker would struggle in it, but this wouldn't be any worse than any others. I also have a PA system you could borrow a speaker of if you wanted to really blast it out for pass the parcel...
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The real answer is a couple of minirigs and a minirig sub.
More realistically, these are pretty decent and can be picked up for £60-£70 or so on ebay https://www.libratone.com/uk/products/speakers/zipp-2/
Looking for a portable bluetooth speaker for a children's party to put some music in a room that's I guess 7-10m x 15-20m.
Have tried to find something to rent or borrow but getting short of options. Don't want to spend a fortune but don't want something that sounds like it's playing underwater through some tissue paper wrapped in tinfoil.