There is a huge selection of detectors available ranging in price from around £100 up to several thousands.
Most “serious” detectorists that I know spend between £500 and £1000 on their detectors.
You can spend a lot more but I’ve not seen people rushing out to buy them.

The simple answer is that you usually get what you pay for.
You can buy a detector from Tandy for under £100 but I don’t know anyone who uses them seriously.

There are a few manufacturers who offer good detectors for between £100 and £200 such as :-
 

Viking (e.g. VK20 or 30)

Whites (e.g. Classic 1)

Garrett (e.g. Treasure Ace 100)

C-Scope

Bounty Hunter (available at Costco and dealers)

I’d like to hear from users of these machines (or dealers) so that I can feed back the consensus to future visitors.

In the £200 to £600 bracket you get more choice. There are models by:

Whites   (e.g. XLT)
Tesoro/Laser  
Fisher  
C-Scope 
Garrett  
Minelab  (e.g. Sovereign)

When I attend rallies or go out hunting with the Weekend Wanders the two most popular models that I see being used by serious detectorists are Minelab's Explorer (now Explorer II) and White's XLT (or DFX). Tesoro's Cortes is also supposed to be a good machine. (Any other suggestions for "top end" machines anyone?)

My first detector was a Whites Classic III which did a reasonable job. I now use a Whites XLT which I like because of its depth, flexibility and for the information that it gives me about the target before I dig it up. This costs around £550 but has paid for itself several times over.

A few other points on choosing your detector:

- Look in the magazines (Treasure hunting and the Searcher) for field test results and dealer advertisements.

- Don't pay the full list prices for detectors - decide what you want and then shop around - do not pay £850 for a DFX

- Talk to dealers - get their opinions - there are links to several dealer sites in the links section.

- Decide whether you want a simple machine (switch on and go) or one that can be tweaked to suit different conditions

- Be clear where you want to detect (e.g. beaches or inland) as different detectors are tailored to specific conditions

2. WHERE CAN I DETECT?

You've got yourself a shiny new detector - you've tested it in the garden and found a threepenny bit - you're now more ambitious - where do you go next?

More or less all land in the UK is owned by someone (farmer, landowner, government) so the most important thing to do is to ask around and get permission to detect on their land. Ted Fletcher's written some good guides on how to do this.

Read the metal detectors code of conduct - it can be obtained from the NCMD site - always get permission before detecting

You usually can't start digging up the local park - I tried it once and kept getting funny looks. I now only detect on private fields.

Initally, don't worry too much about the history of the particular field your friendly local farmer lets you go onto - just see what comes up. This is a valuable part of the learning curve. Once you've tried a few different sites you can get more fussy and do some research. But initially, just get out there and detect.

You can usually detect on beaches in the UK without obtaining specific permission

Join a detecting club. This will not only provide you with somewhere to detect but also enable you to do so in the company of more experienced detectorists. They will be able to pass on valuable advice and help you to identify your finds.