On a single zero roulette table the house advantage is 2.7%. On a double zero roulette table it is 5.26% (7.9% on the five-number bet, 0-00-1-2-3). The house advantage is gained by paying the winners a chip or two (or a proportion of it) less than what it should have been if there was no advantage. Roulette of the sort played in British casinos allows for the player to place 'Even Chance' bets. They are High, Low, Red, Black, Odd and Even. As zero is green and none of these, then none of these bets is paid out as a winner when zero comes up. However the player gets to retain half of their bet. This brings the house edge down to around 1.35%. GGG advice is to play 'even chance' bets if you are new and want to learn. They have the problem of being dull and also offer little chance to win in an evening's play. The normal house advantage from playing the numbers in Roulette is 2.7%. i.e. A single chip placed on a winning number should return 37 chips to break even but it returns 36 instead. The edge is 1/37.

Double Zero or American. The 'double zero' game as played in the U.S and other countries under the guise of 'American Roulette' has a total of 38 numbers ('0', '00' and 1-36). The payout odds are the same as the single zero game and so the edge on a single number is 5.26% i.e. A single chip placed on a winning number should return 38 chips to break even but it returns 36 instead. The edge is 2/38.
As an aside, there are two other differences.

No half stake return or en prison (French game) rule. When either '0' or '00' occurs, all Even Chance bets lose!
A single chip bet covering the four numbers 0,1,2,3 on a single '0' roulette wheel is paid at 8-1 and maintains the 2.7% house edge. Here however, because of the '00', the chip now covers 00,0,1,2,3, which is five numbers and paid out at 6-1. This is a vig. of 7.89%.