Take a look at http://www.growell.co.uk ---this is a good company, prices are OK and the delivery service is excellent, goods are well packaged and gets to your house quickly. They respond to e-mails quickly aswell. I've bought stuff from them several times and the only complaint I have is it takes ages to get all the bubble-wrap off they put lots on LOL!

I've also bought from http://www.hydroponics.co.uk ----good shop.

Fuck the halogens right off, they're useless for growing plants, wrong spectrum and they're too hot.
You won't usually find metal halide in a DIY store, they only sell sodium lamps normally, like security lamps and industrial/factory lighting. They come in 70w, 150w and 250w. It's best to buy from a hydro shop anyway.

P.S- Not really a good idea to ask at a DIY shop for these lamps. I know you could say you need it for an outdoor security lamp or something but you don't know who you're talking to. Best to be safe and have peace of mind.