- Can you do this in person in London? Are there department stores that may carry it? My grandparents prefer this so that they can try and see the filter before
I assume you have seen the upvoted advice on your previous question, which advises you that you might not need this device after all.
I'll assume you have weighed this over, and decided to get the device. Therefore, let's answer your question.
Due to the decent quality of the tap water in London, these devices are probably not all that popular there. Since it's such a huge city, it's probably available in certain shops but the additional requirement of "portable" would probably make it (especially RO) much harder to find.
My grandparents prefer this so that they can try and see the filter before
Unless your grandparents have special knowledge of water purification systems, they don't need to see them before they buy them. The main thing is getting a functional system. Amazon UK has many possible candidates for portable RO systems. That is indeed the easiest way. Check out the reviews before you buy. Amazon even has a nice return policy. Returns with Amazon are very easy, should there be anything unsatisfactory about the product. You could even get a total dissolved solids meter to test the water the product gave you. That way you'll know whether it works or not.
You can get a distiller there too. But I recommend RO. Distillation is wasteful of electricity.