You say it brings in £0.5bn so doubling it would presumably bring kn £1bn. Labour are giving the MoD £6bn to increase defense spending from 2.3% to 2.5% of GDP, so I don't know how Ed Davey thinks an extra £0.5bn from digital services tax is going to find the £21bn needed to go from 2.3% to 3%.
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https://commonslibrary.parliament.uk/research-briefings/sn02783/