The most popular student models are the Buffet B12 and the Yamaha YCL250. Both are very good instruments, which will serve you until you decide to upgrade (and beyond). And when your finished with them, you can sell them for a reasonable amount: I recently sold my old Buffet B12 for £160! The RRPs are £480 (the Buffet) and £420 (the Yamaha), though most shops would sell them much cheaper than this, and you can get second hand ones for bargain prices on ebay. Other manufacturers, such as Leblanc, also produce student models, which are also of good quality.
There has been a recent influx of cheap, often unbranded Chinese models. A few years ago I would have advised that these were best avoided. However, these instruments have really improved in quality recently. I have several students who play on ultra-cheap Chinese clarinets and saxophones, and they are astoundingly good instruments for the money! They all needed a good set up from a professional technician, but this wasn't a problem.
Personally, I would never buy a clarinet of this standard without playing it first. However, some of the bargains on ebay are just too good to miss.
Buffet and Yamaha are the most popular here: Buffet with their "E" series (£853-£1165) and Yamy with their YCL650 (£959). Once again, I would personally choose the Buffet, probably the more expensive E13. The Yamaha is, however, a good instrument. Almost always overlooked is the Leblanc.The Leblanc Esprit, with an RRP of £1295, is the most expensive student clarinet (Though you will rarely find it being sold at RRP).
I wouldn't advise buying pro-level instruments online. Even within brands and models, two instruments can vary greatly in feel (and sometimes even quality!). If you're buying new, go to a shop! However, eBay is a goldmine of second-hand instruments. As before, stick to the main brands, and you won't go far wrong. Maybe see if you can try out some instruments (a friend's, or a shop's of you're feeling cheeky!) to give you an idea of what make you might want to go for.
Bargains can be found in the form of older instruments, such as Boosey & Hawkes' 1010 and 926 models. These instruments can often be a bit "quirky", so are probably only suitable for an experienced clarinetist. I own a pair of 926's, and find them to be brilliant orchestral instruments, with a sound that can at least match modern pro-level models.
I hope this guide is useful, and I may make additions and changes as time goes on.