Most modern airliners nowadays have Datalink (CPDLC) so receiving a clearance in the NAT HLA to divert around weather is not an issue. Been a long time since I was forced to use HF comms. However, you should make all attempts to receive a clearance prior to the event. If unable, then as SAFELIFE has stated you are authorised to follow the Contingency procedure for weather avoidance, whilst still attempting to receive a clearance.