You can't get access to Kindle Unlimited on a Kobo reader. The books in Kindle Unlimited (and all the books available via Amazon) are in the .azw format used by Kindle ereaders, whereas Kobo uses the .epub format.
Both Kindle and Kobo readers have a built in dictionary, although the dictionaries on the Kindle are far better.
If you'd really prefer a Kobo reader, there might be good news on the way for you, as Kobo are apparently getting set to launch a subscription service. When, or even if, it'll come to the UK doesn't seem clear, and how good the selection will be compared to Kindle Unlimited is also unknown. See:
http://the-digital-reader.com/2017/0...ice-kobo-plus/
https://www.mobileread.com/forums/sh...d.php?t=283873
If you get a Kobo ereader, you might not be able to continue to use Kindle Unlimited, but you will be able to borrow library books if your local library has ebooks.