Aye, what Deecie said
We did the same when getting the flight out to Macedonia - think its about 17 miles or something like that from Watford. We pre-booked a taxi - sorry but cannay mind their name, but there's a few if you google and all about the same price
Made sense for us to do this as the flight was 08.20 and you get into Watford at about 06.20 from memory but I suppose if your flight isnt that early, then maybe Euston is more suitable?