I've got an offer from a reputable University in the UK for a PhD in the Humanities. I've previously been in contact with the potential Supervisor. He very kindly invited me to apply but did not reply to the email after confirming I had actually applied. I did send him a follow-up asking for feedback following a notification regarding funding and got an offer a few hours later. Is it okay for me to send a thank you or contact him before I decide to accept or reject the offer. (I've got a couple of other applications still ongoing at equally good Universities etc.) Or is it expected that I should make up my mind first? Thank you.
The best thing to do is send a short email thanking him for the offer and for his support of your application. Explain that you are waiting to hear back from a couple of other universities before making your decision. This is a perfectly reasonable thing to do, and no one would expect you to make a decision immediately.
Congratulations on the offer!