I'm not sure if they can help with the foxing, but Camberwell College of Art (020 7514 6302) does a museum conservation course. Perhaps also try the London College of Communication (School of Printing & Publishing 020 7514 6700) or Intaglio Printmakers Southwark Bridge Road - sorry no numbers.
Hello. I am a printmaker so have to get a lot of my own work framed. It is a problem finding a good framer and it is often extremely costly, particularly, of course, in London - I currently send the majority of my work to a man in Yorkshire who used to work at the Tate! But I have had an old etching framed at Pure & Applied on Bermondsey Street (who do 'conservation grade' framing - i.e. to museum standard - and are Fine Art Trade Guild commended framers and they did an ok job, albeit expensive. Good luck and do report back!
There is an excellent framer in Lower Marsh, Kevin Phillips. Shop doesn't look much from the outside but really knows his stuff. Can supply all types of glass up to museum glass. Worth ringing in advance as he is not always there even if the shop is open. 7928 4944.