You can definitely have fun without drinking. Join societies, go to events, make friends and hang out with them. There's plenty of fun stuff to do in London that don't include drinking.
If you do want to drink, it's your choice though. As long as you're not being peer pressured into it and you do it safely there's nothing wrong with it aside from religious reasons. If you personally disagree with those religious reasons and are living away from home then go for it and drink. Your parents can't control you or give you a curfew if you're not living with them. (If you are living with them then that's a bit tricky and it's probably not the best idea to go clubbing and break curfew, but you can still have a drink at the pub with friends if you feel like it for example.)
Also, in terms of finding a partner... I've got nothing lol. I've never been in a relationship. But make sure that it's what you want and not your parents pressuring you into it. It's ok if you don't get married by 25!! You'll still be really young and have plenty of time to find the right person for you. It's better to not rush into an unhappy marriage than rush and end up marrying someone who doesn't make you happy. I will say that if you're dating for marriage it's probably a good idea to bring up that you're looking for a serious relationship near the beginning so you don't waste your time with people who are only looking for fun y'know?