Any recommendations for a good and accessible thesaurus?

iOS and iPadOS

Hi all,
I am looking for a good and accessible thesaurus to help me when I write up my PhD. I have had a look through and tried the ones listed in this directory but I do not think they are that good.
I am happy to pay for a good one but I do not want to go ahead and pay money if I then find that they are not accessible. Any recommendations please?
Also, I have a free dictionary app and use Siri for words that I cannot remember how to spell but I would like some recommendations for free or paid dictionary apps too that are of course accessible.
I do not know whether UK English and USA English dictionary and thesaurus apps are the same but please note that I am in the UK.

Thank you,