Would this put you off buying a flat?

[Diamondeye]

If one of the cars parked in the communal area had a sign in each window saying “no valuables left inside”.

All white vans have that don't they??

I would have a look at police.uk and see what the reported crimes on the street are. Although try not to get too bogged down in it... everywhere has stuff reported! But you could compare it to where you are now, for example.