It took me a while to track down a company who would supply the excellent series of Casio electronic dictionaries to the UK by post, and who had a credit card Internet ordering system. In the end I found MundoElectro in Spain who have good prices and will deliver to the UK at a reasonable rate. Navigate to Calculadoras y Traductoras then Diccionarios Electronicos. They had four of the models available last time I checked: EW-S100, EW-S200, EW-S3000 and EW-S3100V. See also the Casio Europe site (in Spanish) for the full specifications of these dictionaries.
In late 2010, ordering an EW-S100 and an EW-S3100V from MundoElectro cost me 32 euros + VAT for shipping. There is no import duty within Europe, so these will arrive without any extra fees from the Post Office or ParcelForce -- unlike ordering from the States or Japan. It took about 12 days from ordering for the parcel to arrive -- slower than we're used to in the UK, but it did arrive without problem. (I have no connection with MundoElectro, except that I bought a couple of dictionaries from them.)
About the dictionaries: I've had an EW-S100 for over 6 years and it has been in constant use by both my Peruvian wife and I over the whole of that time. I know that the Oxford dictionaries are also available as iPhone or Android apps, but these Casio dictionaries are much more polished in comparison, and with a greater number of features than the iPhone apps that I've seen, and are also much quicker and easier to use due to the single-function buttons and the hard keypad. Also, be careful because some of the iPhone apps use the 'concise' Oxford dictionary instead of the full one, which has less than half the number of examples, and many of the definitions simplified. Just one note on the EW-S3100V's voice feature, though -- the pronounciation uses American English, which was confusing for my wife learning British pronunciation, so the EW-S3000 would have been just as good for us.
If you're considering one of the cheaper Franklin / Merriam-Webster electronic dictionaries, from my experiences I would say: avoid them at all costs. The cheapest Casio may be double the price, but it will repay that outlay many times over in usefulness.
For one thing, the Merriam-Webster dictionary used by the Franklin doesn't go into much depth or give more than the occasional example. One of the benefits to me of the full Oxford Spanish-English dictionary used by the Casios is the number of examples showing typical usage of the word in everyday phrases. It is sometimes hard to absorb the meanings from a definition, but a practical example makes things much easier to grasp.
Also, the user interface of the Franklin dictionary I owned (DBE-1490) is really painful in comparison to the Casio. (It would be hard to exaggerate the scale of the difference.) Never mind the squidgy rubber keyboard or the tiny screen -- which you might expect from a cheaper model -- those are only the start of the differences.
Whereas the Casio typically has a single-purpose button that takes you straight to the function you need, the Franklin will require you to navigate through one or more levels of menus, or guess that a particular button has some obscure meaning in that context. (For example, I've seen people claim that the Franklin only conjugates verbs in the first person singular -- but actually if you experiment pressing different buttons, you can find your way onto a page with 2nd and 3rd person as well -- but it is so obscure and hard to use that it might as well not be there at all for the average user.)
The Franklin I owned seemed to have an immense number of selling points on paper, but in practice it was just too painful and frustrating to use anything more than the basics. For example, whereas the Casio immediately gives you a list of words starting with the letters that you have typed so far, the Franklin requires you to enter the whole word, or if you want a list, to enter a few letters and then a wildcard and then press Enter to bring up a list which you can then select a word from. That's several keypresses extra and a number of seconds of your life wasted in frustration. With the Casio, if you can see the word you're looking for after pressing 3 letters, you can just hit enter and you're straight there.
In addition, there seemed to be no care for elegance or consistency in the interface design of the Franklin. The ascenders and descenders of letters on adjacent lines touch -- as if you are reading a book typeset with no leading -- and letters with accents are squashed down compared to non-accented letters. Different parts of the interface seem to have been written by different people to different standards, and none of it really seems to hold together. It is like a maze of incompatible sub-programs, none of them really quite polished, and none of them very well integrated with the others, if at all. In some ways it reminded me of using a suite of badly designed MS-DOS programs, or maybe the experience of programming a VHS recorder with a remote in the 80s or 90s, or something like that. It is as if they had a floor of low-paid programmers whose task was to tick a number of marketing checkboxes in the least time possible, with no interest in style or elegance or any kind of overall product vision.
To me, the Casio is the complete opposite -- clean design and a good user experience come first and the dictionary does all that it can to make your use of it as immediate and responsive as possible, getting you to the information you need quickly whilst avoiding being over-complicated, obscure or hard to learn.
However, it seems like even Casio have felt it necessary to compete on the marketing checkbox-ticking front to some extent, by adding phrasebooks and voice and so on to later models. I don't think that these add much to the dictionary, to be honest -- the voice seems to be mostly American English, or a synthetic voice for the other languages -- and from what I've heard, I wouldn't trust it as the primary source for learning a pronunciation. However, at least the phrasebook may be useful as another source of examples, as it can be searched by phrases (e.g. "leave&tip" or "again&tomorrow"), although it isn't integrated into the multiple dictionary phrase search function like the other dictionaries.
I think it is best to concentrate on the Casio's ease of use within the 4-5 core dictionaries and not worry too much about all the extra marketing-driven gizmos, because it will be the core dictionaries that you'll find yourself coming back to again and again over the years of learning and improving your use of the Spanish or English language.