I wish to travel to south africa in February.my passport (german)expires in march.if I buy the ticket and return before the due expiry date can I travel?

There is no requirement when entering the UK other than that your passport needs to be valid.
http://www.ukba.homeoffice.gov.uk/customs-travel/Enteringtheuk/arrivingatukborder/

However, your passport needs to be valid for 30 days after you exit South Africa.

http://www.fco.gov.uk/en/travel-and-living-abroad/travel-advice-by-country/sub-saharan-africa/south-africa#entryRequirements
Entry Requirements - Passport validity
You must hold a valid passport to enter South Africa. Your passport must be valid for a minimum period of 30 days from the date of exit from South Africa. South African authorities state officially that only one blank passport page is required for entry. However, visitors have been refused entry and sent back to the UK as some officials are insisting on two blank pages. We advise that you have two blank pages in your passport on arrival.

Yes you can but your passport has to be valid.