Something I have not come across here in Thailand and am struggling to find is the standard hot water cylinders that are so common in the UK, made from copper. Are they available here?

Tank heaters are available, they are usually intended for horizontal mounting on the wall and white in colour.

The bigger DIY places should have them.

Like these https://www.directtoshop.com/category/12154 of course, all "out of stock" at HomePro

Virtually no demand for copper hot water cylinders

If you get a stainless steel water tank and stick somewhere out in the sun you'll get free hot water. Sometimes too hot.

I am of the belief that most every product is in Thailand. However some chemicals are never allowed here and many higher end models are not normally imported into Thailand. Sometimes I have to search in Thai to find some items I want. At the Architect Expo held in Bangkok no shortage of home building products you would not see on the shelve of a modern trade store or builders merchants store. I always consider who is going to do the INSTALLATION, who can really come to do the warranty repair, and who has actual stock of parts before I order any special item. Price plays a huge role for me on most products. If a Thai version, can meet my needs, then I'll pass on the import. Water heaters of all sorts are in Thailand including brands I recall from the USA. http://www.stiebeleltronasia.com/showtype.php?productcateid=1 I have seen Rheem water heaters in Bangkok http://www.j7eng.com/HTML/rheem-electric.html The OP might have luck using google and the brand he knows from the UK and see if they have a distributor in Thailand. Often a distributor will carry expensive imported products and some less expensive similar product brands of the same type. I have person experience in how Stiebel Eltron and Toshiba each handle multi point electric water heater warranty repairs in Buriram Province.