You may use planes/trains/cars, but no privately chartered planes or helicopters.
You must take in to account how busy the roads will be at the time.
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If you count time to get through airport security/time spent in the airport etc. trains/driving would be faster.
The fastest capital city you can reach would be Paris, as well as Cardiff, capital of Wales if you're looking inside the UK. On a Wednesday afternoon you could leave that postcode address at 12:18pm and arrive in Paris within 3 hours and 41 minutes, via trains & Eurostar (including 30 minute check in time for Eurostar) (Enfield to Pancreas at 12:18-12:59, Eurostar from 13:31)
For Cardiff you would leave at 12:12 and be there for 15:23 via trains, 3hr 11 minutes, but since many class the UK as one 'country' instead of being made up of several countries I'm not sure if this is what you're after. (source)