Prime Minister Theresa May said:

Enda has been a strong and consistent friend to the UK and I want to thank him for all he has done to maintain the unique and close spirit of cooperation between our two nations, which has gone from strength to strength during his time as Taoiseach.

On behalf of the UK, I wish him all the very best for the future and look forward to working with his successor, when in place.

https://www.gov.uk/government/news/pm-statement-following-enda-kennys-resignation-17-may-2017