Using syringes that have previously been used by someone else can easily lead to infection with HIV. If a group is sharing the same cup of water to clean out (flush) syringes or other equipment, there is also a risk that infected blood may be present.
Needle exchanges can be found in most parts of the UK (it could be a drop-in centre, a mobile delivery service or a pharmacy-based scheme). They should be able to advise you where your local drug service is, and the drug service will then be able to tell you where you can get injecting equipment. Most needle exchange services will also offer safer injecting advice, primary healthcare, free condoms and safer sex information.