As far as I know, the UK legislation required in the UK to hold European elections has been revoked. If Brexit gets delayed and the UK is forced to hold European elections, could the elections be held without the UK legislation that has been revoked? If they can't, would it have to be re-introduced as new (UK) legislation (and go through the full process) or could it be fast tracked?
The legislation enacting European elections has not been revoked yet. It was due to end on March 29th, but that looks unlikely now and if the exit date is delayed then the legislation will remain in force until the new date.