By overnight I mean you would arrive late evening and leave in the morning, so you would have to find somewhere to sleep (hotel, hostel or park bench
) depending on your financial resources.
The
DB booking engine is in English and is fairly easy to use. However, sometimes you have to play around with the connections to find the cheaper fares. If you need any specific help, just ask.
Your travel in Germany would be mainly on ICE trains which is 2-2 across seating in 2nd class and 2-1 across in 1st class. In 1st class, apart from the extra and less crowded space, you don't get much more (a German newspaper sometimes and at-seat food and drink service by the conductor, but you have to pay for it). Personally, I think it is worth it for long journeys, especially at weekends when many trains are very busy. You can also use the DB lounge at Leipzig (and at connecting stations eg Frankfurt and Cologne).
Of course, a 1st class ticket also gets you Leisure Select on Eurostar, which is also 2-1 seating, and catering is included.
The prices I quoted include the reservations on Eurostar, but not on German trains when not compulsory. (You can pay extra for them if you want them).
Edit to update: I guess that leaving Leipzig on Saturday morning on the return would be too early for you. Then you can do the journey in one day:
Leipzig dep 09:11, London StP arr 19:22. Fares: €49 2nd, €99 1st.