I think invading is too strong a term here because it implies force and violence. As your link indicates, squatters can only move in to a building which is 'unoccupied'.
Local car boot sales, private adverts in shop windows.
If you are in or near London, Brick Lane market may still be a possibility, although Wikipedia says it's not so notorious as it once was.
Consider challenging the insurer's decline of your claim, particularly if the bike was expensive.