London SID Aerad plates have a note in bold saying "DO NOT Climb above SID levels until instucted by ATC".
If 5000' is your initial-cleared/blocked altitude then you maintain 5000' until further climb is instruction by radar.
Once identified under radar control you can't initiate a climb simply because it says so on your SID plate. You must get clearance from ATC before leaving your cleared altitude/level.