I am a second year PhD student. I have the opportunity to go abroad to learn some new lab techniques. The internship would last 1 month and I have been accepted by the host institution. The problem is... my research allocation would not cover the costs. Anybody knows about funding I could apply for? research societies or charities?

The Royal Society offer travel grants:

http://royalsociety.org/funding.asp?id=2348

and depending on your discipline, a lot of smaller societies such as the Society for General Microbiology will offer the same:
http://www.sgm.ac.uk/grants/smtg.cfm


If you're funded by one of the big research council like BBSRC, check whether they will offer a similar grant.

I got funding to travel to an international conference from a couple of associations I am a member of. Alot of societies/associations have student travel grants and bursaries so it might be worth contacting them if you are a member.