I have owned an All American 921 21 quart pressure canner since about 5 years.  Bought it off Amazons' US site and paid to have it shipped to the UK, hand carried to Hungary from there.  Think that it will cost something like 220 pds to do this today, i.e. to get one in the UK.  Cheaper lightweight stainless steel versions with a gasket are probably 50 - 80 pds less, but I wanted one that works without a gasket for longevity. Holds up to 14 750 - 800 ml jars at a time.

It's a magic device, and I use it frequently, especially to jar meats, fish, stocks, soups and some prepared foods (best baked beans ever).  Part of the reason for getting it was our second freezer was on its way out and the cost to buy the canner was about the same as a replacement freezer.  The other part was that I liked the idea of prepared food that you could eat without having to think, dig about for, thaw, microwave.  With this you just crack the jar - a lot of the time, I just heat the jar in a pan of simmering water for about 15 minutes, crack the lid and serve.

I researched quite extensively before I bought the device, and learned enough about the risks involved that I take them very seriously.  I only follow USDA approved recipes and cooking methods - easily available online.  Apparently it costs up to $10k to fully test a new recipe - you need to be cracking jars from different batches for over a year and testing them in a lab, so not a trivial exercise.  In the USA the Department of Agriculture contracts University laboratories to do this. 

A basic recipe book comes with the canner, a fuller one is available from Blue Ball who are the biggest jar suppler in the US.  The risks of "free-styling" outside tested methodology are that you can poison yourself, your family or anyone else with whom you share the food.  Botulism spores survive extensive boiling, and thrive in a low oxygen, low-acid environment - which is exactly what you have in a jar of preserved meat or veg.  Botulin, which is the poison produced, is odourless and tasteless, often fatal and always very nasty - as in minimum weeks in hospital, months to recovery and very likely lasting damage.  What kills the spores is an extended period at over 110 celsius, which is what a pressure canner does and what a hot water bath, by the laws of physics, cannot.  Preserving meats or low-acid veg in a hot water bath is playing Russian roulette with at least 2 chambers loaded.

You don't need one if all / most of what you preserve is fruit or tomato based - these are acid enough that botulism cannot survive, but if you have a lot of veg, meat or fish to process the canner is well worthwhile, if you can get the jars easily.  New 750 ml jars with screw-top lids are about 30 p each here, but I bet much more in the UK.  I would point out that my partners' parents keep pigs and poultry and have a 1/2 acre vegetable plot, and we spend each summer by a lake with fish traps in the water, so I get good use out of the investment.