I like these style of laundry bags for bras. I have some the same size but not the same manufacturer and they easily fit G cup bras while being sturdy enough to actually protect them. If you can find something like this in the UK, I would give it a try. The size you want is 20 cm x 20 cm:
https://www.amazon.com/Polecasa-Large-Mesh-Laundry-Bag/dp/B089ZVNL8K/

If you can't find it in the UK, I might try the laundry bags made for shoes. They tend to have more structure and would be more protective I think. The size looks like it would work:
https://www.amazon.co.uk/Bncxdc-Laundry-Cylinder-Sneakers-15-7x7-48inch/dp/B0BZ84GPDP/