I need to travel to Guangzhou in China by the cheapest possible means.
 I think the cheapest way to do this is to fly to Hong Kong and then
take a train to Guangzhou. During my trip I will also visit a friend
in Kunming so I could fly to Kunming from the UK if cheaper.  I don't
mind the inconvenience of flying to other places in China and then
taking a train to Guangzhou if this would work out significantly
cheaper.  I am on a very tight budget as I am a student and simply
need the cheapest option.  I had a friend who says he took a return
flight to Hong Kong from the UK for 180 pounds; however he can't
remember who he booked it through.  Ideally I want to travel to China
this month for a minimum of 1 week or a maximum of 1 month.  The
precise travel dates will depend on how cheaply I can arrange flights.
 Also if anybody has any information on cheap hostel/ hotel
accommodation in Guangzhou or Kunming I would be very grateful.

In your answer is would be helpful if you could provide direct links
to the cheapest flight deals.

When I was a student, there were courier flights, where the flight was
heavily subsidised by a courier who took your cargo space. Downside
you only have hand luggage and have to travel at short notice as they
define the time of travel.
However, I'm not sure if this sort of scheme operates these days.

Good luck

regards lot-ga