(1) I would highly recommend www.bookfinder.com ! That website checks a huge number of online booksellers for all editions of books (or as you specify in your search). I've ended up buying books from Amazon, Abe Books, and a few others after searches there. I would suggest you search one title at a time, or you will get overwhelmed. One title might give you a list of 50 search results from all the different ways that authors' names were spelled in the listings (Joe Schmoe; J. Schmoe; Schmoe; J Schmoe; J Schmoe and B. Smith; B Smith; and so on), publishers' names and variations, publishing dates, etc. It looks like there is a new feature to combine search results, so that will make things easier.
So, for example, Goodreads says that the first book is "Flight from the Dark." Using that new feature and using the advanced search with that title and just "Dever" as the author name, I get this book result without limiting by publisher or anything. It looks like the cheapest copy is $6.43 from Alibris books. You will want to revise your search by your ship-to country (in the UK?) and anything else you find important.
(2) Check out EBay. Sometimes with series you are able to find someone who is selling a lot comprised of most books of an old series. This is especially true for manga, but I have seen it for old fantasy book series as well. If your local version of EBay is not that big, try the US version EBay.com . It looks like a lot of sellers will ship to non-US countries, and you can always message them and ask. You can also have new search results sent to your email.
Searching for your series by title and author surname (try main character name as well!): link to Lone Wolf Dever
Lot of 18 books for $130
Lot of 12 books for $80
(3) Long-shot: If these books were popular in your area when you were a child, try your local equivalent of Craigslist/Kijiji/Gumtree. Look at sale and free listings, and try placing your own Wanted listing. If smoke-free is important to you, be sure to specify that!
(4) Try your local library! And see if your library is part of a larger network that will let you borrow/place on hold books to be shipped to your library for pickup.
(1 and 2) As you start gathering information on buying these books, you will want to put together a spreadsheet to see what is the cheapest way to buy all of them. All individually? One complete EBay lot? One small EBay lot, and the rest purchased separately? Be sure to take shipping costs into account. I had to do this recently when buying a series for my sister. For me, the answer was one small lot and a few individual books.
(Last) Finally, as you wait and plan and plot, read a few online via OpenLibrary (signup required)! You might be able to hunt down more, but here are three:
Joe Dever at OpenLibrary
tl;dr - Try bookfinder.com, ebay.com, your local classifieds, your local library, and openlibrary.org