A global grid would be an immense coordination problem with tremendous costs. Some form of storage local to each grid is likely to be more cost effective. There are so many storage options, something should be appropriate as the need becomes real: pumped hydro / other mechanical, battery, synthgas (maybe), heat, pressure.
Repeat that cable 9.6 times over 3,459 miles from London to New York would be 66% efficient and cost €5.8b.
If energy is $0.05/kwh in London and you can sell it around the retail rate of $0.12/kwh in New York, your cable will make around $20k/hour ($0.028/kwh,after cable losses). Your cable will pay for itself after 33 years of complete capacity saturation.
With free energy at one side and retail prices on the other, the payoff time narrows to less than 8 years.
This calculation leaves out so much but it was a fun thought experiment.