Avoid Holland and Barrett.. Extremely over priced, the vast majority of us on TSR get our stuff from myprotein, cheap and good quality.
It depends what your needs are, how good your diet is and how active you are. Assuming you are fit and healthy and have no actual medical problems, combined with a decent diet, you really don't need much. Personally I take a multivit(alpha men) fish oil, and zinc everyday. Then supplement whey and taurine when needed.
A general multi vitamin and mineral one -a -day should be fine unless you have a terrible diet. Boots have a huge range. There is one called "Well Teen" which is quite expensive but has everything you need including a decent dose of Vitamin D which a lot of people are low in as you get it from the sun. (If you're an outdoorsy person this may not apply to you but many of us don't get enough vitamin D)
Why take vitamin tablets at all? Vitamin deficiencies are quite rare in the UK, and almost never occur in people with a balanced diet (except with serious underlying diseases eg some bowel diseases or a s a side effect of medications). And furthermore it's probable that a balanced diet has additional health benefits beyond its vitamin content, so that you'll gain greater benefits from a balanced diet than you will from an unhealthy diet + a multivitamin.