The basic kryptonite d lock used to be sold secure gold rated, oddly the new one is only silver. Available online with a length of cable to runt through the front wheel too for about £30.
I’m of the opinion that if they want your bike they’ll have it in a few seconds so:
1) don’t ride arround town on a nice bike, get something secondhand for £100 (you can get some really nice bikes for £100! I’ve got a charge plug CX bike* and a charge cooker 29er for arround that!). It doesn’t matter if you have nowhere to store your n+1 the whole point is it wont get nicked left on the street locked to some railings.
2) Insure it. They will either just ask for a d-lock or some standard. Meet that and you’re covered.
*I’ve added about £250 of dynamo wheels and lights to it, but it’s still invisible to scrotes.