Do note that the southbound GNR and Victoria line platforms at Highbury & Islington are literally next to each other, so that particular option shouldn't present any problems for you. Exiting the Underground at Euston is a bit of a zoo of people but is step-free. So I would say that if New Barnet is good for you, then the rest of the way to the Virgin platforms at Euston should be relatively easy, as long as it's not rush hour.

I would, if possible, avoid the journey by road from Barnet to Euston - it's not the most enjoyable.