To have unlimited YouTube streaming you would need to purchase a video pass for an extra £7/mo.
For what's included in which pass see https://www.vodafone.co.uk/mobile/pay-monthly/vodafone-passes or https://www.voxi.co.uk/passes
Pay particular attention to the terms and conditions, there are MANY exemptions to the passes which could see someone very quickly using ALL of their purchased data allowance and thus be no longer able to use their pass data without first purchasing additional data at frankly extortionate rates.
Pass data overseas also is not unlimited. You'll need to check with support for the current overseas fair usage policy on the passes, (I don't know it, especially the Voxi specific one, and I'm too lazy to go looking).
Ah well, it looks like they have just wasted some more money in sending out a sim to me, as well as the OP.
Havnig said that, it's probably not too difficult to get creative to meet the age restiction - if you know what I mean
I did query the possible age discrimination, but the process of taking it forward via the relevant authority required more effort than I could be bothered to put into it.