there is really no point trying to get a taxi from the astoria to take you to euston station. leave the astoria go left along charing cross road, and get on the underground at tottenham court road, then go on the northern line 3 stops (north bound) till you reach euston. that takes about 12 i think, from leaving the astoria to the doors of euston, taxi's aren't reliable in london, cos they take you the long way and they cost a fucking bomb, bets off getting an all zones travelcard that goes for the underground and overground rail links, it is cheaper and more reliable.
beleive me i hav experienced trains and taxi's both.
go with the train.