OP, for medicine ECs are useful. First of all, they show that you're a well-rounded person and can cope with doing lots of other things as well as just getting the grades you need. However that doesn't mean you should do lots of ECs just to please the universities! For example KCL has a list of things they look for in their applicants, and ECs are there. Universities have various methods of selecting candidates to interview and there are universities which will reject people who don't meet EC-related criteria. Obviously if you know this will happen, you shouldn't apply to that particular university- but many people do.
Also, unlike what nuodai said, volunteering in oxfam can be useful- it just depends on what you have gained from it and how you present it. For example, it can show that you have experience of interacting with people, bearing responsibility etc. Also volunteering in a hospice is a form of work experience.
As for your 2Bs (provided you satisfy the requirements for English Lit/Lang, Science or Maths), they won't be a problem unless your other grades aren't so good/or you don't do particulary well in the UKCAT/BMAT (I have one B at GCSE and it hasn't done me any harm)- but all of this depends on which universities you apply to.
Some universities place a lot of emphasis on ECs and work experience, while others don't. Similary some universites don't actually care about Medlink or similar courses at all. They all have different entrane requirements, and it would be best for you to find them out before applying.